clean install

web/core/modules/language/src/Annotation/LanguageNegotiation.php

@@ -0,0 +1,77 @@
+<?php
+
+namespace Drupal\language\Annotation;
+
+use Drupal\Component\Annotation\Plugin;
+
+/**
+ * Defines a language negotiation annotation object.
+ *
+ * Plugin Namespace: Plugin\LanguageNegotiation
+ *
+ * For a working example, see
+ * \Drupal\language\Plugin\LanguageNegotiation\LanguageNegotiationBrowser.
+ *
+ * @see \Drupal\language\LanguageNegotiator
+ * @see \Drupal\language\LanguageNegotiationMethodManager
+ * @see \Drupal\language\LanguageNegotiationMethodInterface
+ * @see hook_language_negotiation_info_alter()
+ * @see plugin_api
+ *
+ * @Annotation
+ */
+class LanguageNegotiation extends Plugin {
+
+  /**
+   * The language negotiation plugin ID.
+   *
+   * @var string
+   */
+  public $id;
+
+  /**
+   * An array of allowed language types.
+   *
+   * If a language negotiation plugin does not specify which language types it
+   * should be used with, it will be available for all the configurable
+   * language types.
+   *
+   * @var string[]
+   *   An array of language types, such as the
+   *   \Drupal\Core\Language\LanguageInterface::TYPE_* constants.
+   */
+  public $types;
+
+  /**
+   * The default weight of the language negotiation plugin.
+   *
+   * @var int
+   */
+  public $weight;
+
+  /**
+   * The human-readable name of the language negotiation plugin.
+   *
+   * @ingroup plugin_translatable
+   *
+   * @var \Drupal\Core\Annotation\Translation
+   */
+  public $name;
+
+  /**
+   * The description of the language negotiation plugin.
+   *
+   * @ingroup plugin_translatable
+   *
+   * @var \Drupal\Core\Annotation\Translation
+   */
+  public $description;
+
+  /**
+   * The route pointing to the plugin's configuration page.
+   *
+   * @var string
+   */
+  public $config_route_name;
+
+}

web/core/modules/language/src/Attribute/LanguageNegotiation.php

@@ -0,0 +1,54 @@
+<?php
+
+namespace Drupal\language\Attribute;
+
+use Drupal\Component\Plugin\Attribute\Plugin;
+use Drupal\Core\StringTranslation\TranslatableMarkup;
+
+/**
+ * Defines a language negotiation attribute object.
+ *
+ * Plugin Namespace: Plugin\LanguageNegotiation
+ *
+ * For a working example, see
+ * \Drupal\language\Plugin\LanguageNegotiation\LanguageNegotiationBrowser.
+ *
+ * @see \Drupal\language\LanguageNegotiator
+ * @see \Drupal\language\LanguageNegotiationMethodManager
+ * @see \Drupal\language\LanguageNegotiationMethodInterface
+ * @see hook_language_negotiation_info_alter()
+ * @see plugin_api
+ */
+#[\Attribute(\Attribute::TARGET_CLASS)]
+class LanguageNegotiation extends Plugin {
+
+  /**
+   * Constructs an LanguageNegotiation attribute.
+   *
+   * @param string $id
+   *   The language negotiation plugin ID.
+   * @param \Drupal\Core\StringTranslation\TranslatableMarkup $name
+   *   The human-readable name of the language negotiation plugin.
+   * @param string[]|null $types
+   *   An array of language types, such as the
+   *    \Drupal\Core\Language\LanguageInterface::TYPE_* constants.
+   *   If a language negotiation plugin does not specify which language types it
+   *   should be used with, it will be available for all the configurable
+   *   language types.
+   * @param int $weight
+   *   The default weight of the language negotiation plugin.
+   * @param \Drupal\Core\StringTranslation\TranslatableMarkup|null $description
+   *   The description of the language negotiation plugin.
+   * @param string|null $config_route_name
+   *   (optional) The route pointing to the plugin's configuration page.
+   */
+  public function __construct(
+    public readonly string $id,
+    public readonly TranslatableMarkup $name,
+    public readonly ?array $types = NULL,
+    public readonly int $weight = 0,
+    public readonly ?TranslatableMarkup $description = NULL,
+    public readonly ?string $config_route_name = NULL,
+  ) {}
+
+}

web/core/modules/language/src/Config/LanguageConfigCollectionNameTrait.php

@@ -0,0 +1,46 @@
+<?php
+
+namespace Drupal\language\Config;
+
+/**
+ * Provides a common trait for working with language override collection names.
+ */
+trait LanguageConfigCollectionNameTrait {
+
+  /**
+   * Creates a configuration collection name based on a language code.
+   *
+   * @param string $langcode
+   *   The language code.
+   *
+   * @return string
+   *   The configuration collection name for a language code.
+   */
+  protected function createConfigCollectionName($langcode) {
+    return 'language.' . $langcode;
+  }
+
+  /**
+   * Converts a configuration collection name to a language code.
+   *
+   * @param string $collection
+   *   The configuration collection name.
+   *
+   * @return string
+   *   The language code of the collection.
+   *
+   * @throws \InvalidArgumentException
+   *   Exception thrown if the provided collection name is not in the format
+   *   "language.LANGCODE".
+   *
+   * @see self::createConfigCollectionName()
+   */
+  protected function getLangcodeFromCollectionName($collection) {
+    preg_match('/^language\.(.*)$/', $collection, $matches);
+    if (!isset($matches[1])) {
+      throw new \InvalidArgumentException("'$collection' is not a valid language override collection");
+    }
+    return $matches[1];
+  }
+
+}

web/core/modules/language/src/Config/LanguageConfigFactoryOverride.php

@@ -0,0 +1,229 @@
+<?php
+
+namespace Drupal\language\Config;
+
+use Drupal\Core\Cache\CacheableMetadata;
+use Drupal\Core\Config\ConfigCollectionInfo;
+use Drupal\Core\Config\ConfigCrudEvent;
+use Drupal\Core\Config\ConfigFactoryOverrideBase;
+use Drupal\Core\Config\ConfigRenameEvent;
+use Drupal\Core\Config\StorageInterface;
+use Drupal\Core\Config\TypedConfigManagerInterface;
+use Drupal\Core\Language\LanguageDefault;
+use Drupal\Core\Language\LanguageInterface;
+use Symfony\Contracts\EventDispatcher\EventDispatcherInterface;
+use Symfony\Component\EventDispatcher\EventSubscriberInterface;
+
+/**
+ * Provides language overrides for the configuration factory.
+ */
+class LanguageConfigFactoryOverride extends ConfigFactoryOverrideBase implements LanguageConfigFactoryOverrideInterface, EventSubscriberInterface {
+
+  use LanguageConfigCollectionNameTrait;
+
+  /**
+   * The configuration storage.
+   *
+   * Do not access this directly. Should be accessed through self::getStorage()
+   * so that the cache of storages per langcode is used.
+   *
+   * @var \Drupal\Core\Config\StorageInterface
+   */
+  protected $baseStorage;
+
+  /**
+   * An array of configuration storages keyed by langcode.
+   *
+   * @var \Drupal\Core\Config\StorageInterface[]
+   */
+  protected $storages;
+
+  /**
+   * The typed config manager.
+   *
+   * @var \Drupal\Core\Config\TypedConfigManagerInterface
+   */
+  protected $typedConfigManager;
+
+  /**
+   * An event dispatcher instance to use for configuration events.
+   *
+   * @var \Symfony\Contracts\EventDispatcher\EventDispatcherInterface
+   */
+  protected $eventDispatcher;
+
+  /**
+   * The language object used to override configuration data.
+   *
+   * @var \Drupal\Core\Language\LanguageInterface
+   */
+  protected $language;
+
+  /**
+   * Constructs the LanguageConfigFactoryOverride object.
+   *
+   * @param \Drupal\Core\Config\StorageInterface $storage
+   *   The configuration storage engine.
+   * @param \Symfony\Contracts\EventDispatcher\EventDispatcherInterface $event_dispatcher
+   *   An event dispatcher instance to use for configuration events.
+   * @param \Drupal\Core\Config\TypedConfigManagerInterface $typed_config
+   *   The typed configuration manager.
+   * @param \Drupal\Core\Language\LanguageDefault $default_language
+   *   The default language.
+   */
+  public function __construct(StorageInterface $storage, EventDispatcherInterface $event_dispatcher, TypedConfigManagerInterface $typed_config, LanguageDefault $default_language) {
+    $this->baseStorage = $storage;
+    $this->eventDispatcher = $event_dispatcher;
+    $this->typedConfigManager = $typed_config;
+    // Prior to negotiation the override language should be the default
+    // language.
+    $this->language = $default_language->get();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function loadOverrides($names) {
+    if ($this->language) {
+      $storage = $this->getStorage($this->language->getId());
+      return $storage->readMultiple($names);
+    }
+    return [];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getOverride($langcode, $name) {
+    $storage = $this->getStorage($langcode);
+    $data = $storage->read($name);
+
+    $override = new LanguageConfigOverride(
+      $name,
+      $storage,
+      $this->typedConfigManager,
+      $this->eventDispatcher
+    );
+
+    if (!empty($data)) {
+      $override->initWithData($data);
+    }
+    return $override;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getStorage($langcode) {
+    if (!isset($this->storages[$langcode])) {
+      $this->storages[$langcode] = $this->baseStorage->createCollection($this->createConfigCollectionName($langcode));
+    }
+    return $this->storages[$langcode];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getCacheSuffix() {
+    return $this->language ? $this->language->getId() : NULL;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getLanguage() {
+    return $this->language;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setLanguage(?LanguageInterface $language = NULL) {
+    $this->language = $language;
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function installLanguageOverrides($langcode) {
+    /** @var \Drupal\Core\Config\ConfigInstallerInterface $config_installer */
+    $config_installer = \Drupal::service('config.installer');
+    $config_installer->installCollectionDefaultConfig($this->createConfigCollectionName($langcode));
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function createConfigObject($name, $collection = StorageInterface::DEFAULT_COLLECTION) {
+    $langcode = $this->getLangcodeFromCollectionName($collection);
+    return $this->getOverride($langcode, $name);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function addCollections(ConfigCollectionInfo $collection_info) {
+    foreach (\Drupal::languageManager()->getLanguages() as $language) {
+      $collection_info->addCollection($this->createConfigCollectionName($language->getId()), $this);
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function onConfigSave(ConfigCrudEvent $event) {
+    $config = $event->getConfig();
+    $name = $config->getName();
+    foreach (\Drupal::languageManager()->getLanguages() as $language) {
+      $config_translation = $this->getOverride($language->getId(), $name);
+      if (!$config_translation->isNew()) {
+        $this->filterOverride($config, $config_translation);
+      }
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function onConfigRename(ConfigRenameEvent $event) {
+    $config = $event->getConfig();
+    $name = $config->getName();
+    $old_name = $event->getOldName();
+    foreach (\Drupal::languageManager()->getLanguages() as $language) {
+      $config_translation = $this->getOverride($language->getId(), $old_name);
+      if (!$config_translation->isNew()) {
+        $saved_config = $config_translation->get();
+        $storage = $this->getStorage($language->getId());
+        $storage->write($name, $saved_config);
+        $config_translation->delete();
+      }
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function onConfigDelete(ConfigCrudEvent $event) {
+    $config = $event->getConfig();
+    $name = $config->getName();
+    foreach (\Drupal::languageManager()->getLanguages() as $language) {
+      $config_translation = $this->getOverride($language->getId(), $name);
+      if (!$config_translation->isNew()) {
+        $config_translation->delete();
+      }
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getCacheableMetadata($name) {
+    $metadata = new CacheableMetadata();
+    if ($this->language) {
+      $metadata->setCacheContexts(['languages:language_interface']);
+    }
+    return $metadata;
+  }
+
+}

web/core/modules/language/src/Config/LanguageConfigFactoryOverrideInterface.php

@@ -0,0 +1,63 @@
+<?php
+
+namespace Drupal\language\Config;
+
+use Drupal\Core\Config\ConfigFactoryOverrideInterface;
+use Drupal\Core\Language\LanguageInterface;
+
+/**
+ * Defines the interface for a configuration factory language override object.
+ */
+interface LanguageConfigFactoryOverrideInterface extends ConfigFactoryOverrideInterface {
+
+  /**
+   * Gets the language object used to override configuration data.
+   *
+   * @return \Drupal\Core\Language\LanguageInterface
+   *   The language object used to override configuration data.
+   */
+  public function getLanguage();
+
+  /**
+   * Sets the language to be used in configuration overrides.
+   *
+   * @param \Drupal\Core\Language\LanguageInterface $language
+   *   The language object used to override configuration data.
+   *
+   * @return $this
+   */
+  public function setLanguage(?LanguageInterface $language = NULL);
+
+  /**
+   * Get language override for given language and configuration name.
+   *
+   * @param string $langcode
+   *   Language code.
+   * @param string $name
+   *   Configuration name.
+   *
+   * @return \Drupal\Core\Config\Config
+   *   Configuration override object.
+   */
+  public function getOverride($langcode, $name);
+
+  /**
+   * Returns the storage instance for a particular langcode.
+   *
+   * @param string $langcode
+   *   Language code.
+   *
+   * @return \Drupal\Core\Config\StorageInterface
+   *   The storage instance for a particular langcode.
+   */
+  public function getStorage($langcode);
+
+  /**
+   * Installs available language configuration overrides for a given langcode.
+   *
+   * @param string $langcode
+   *   Language code.
+   */
+  public function installLanguageOverrides($langcode);
+
+}

web/core/modules/language/src/Config/LanguageConfigOverride.php

@@ -0,0 +1,105 @@
+<?php
+
+namespace Drupal\language\Config;
+
+use Drupal\Core\Cache\Cache;
+use Drupal\Core\Config\ConfigCollectionEvents;
+use Drupal\Core\Config\ConfigCrudEvent;
+use Drupal\Core\Config\StorableConfigBase;
+use Drupal\Core\Config\StorageInterface;
+use Drupal\Core\Config\TypedConfigManagerInterface;
+use Symfony\Contracts\EventDispatcher\EventDispatcherInterface;
+
+/**
+ * Defines language configuration overrides.
+ */
+class LanguageConfigOverride extends StorableConfigBase {
+
+  use LanguageConfigCollectionNameTrait;
+
+  /**
+   * The event dispatcher.
+   *
+   * @var \Symfony\Contracts\EventDispatcher\EventDispatcherInterface
+   */
+  protected $eventDispatcher;
+
+  /**
+   * Constructs a language override object.
+   *
+   * @param string $name
+   *   The name of the configuration object being overridden.
+   * @param \Drupal\Core\Config\StorageInterface $storage
+   *   A storage controller object to use for reading and writing the
+   *   configuration override.
+   * @param \Drupal\Core\Config\TypedConfigManagerInterface $typed_config
+   *   The typed configuration manager service.
+   * @param \Symfony\Contracts\EventDispatcher\EventDispatcherInterface $event_dispatcher
+   *   The event dispatcher.
+   */
+  public function __construct($name, StorageInterface $storage, TypedConfigManagerInterface $typed_config, EventDispatcherInterface $event_dispatcher) {
+    $this->name = $name;
+    $this->storage = $storage;
+    $this->typedConfigManager = $typed_config;
+    $this->eventDispatcher = $event_dispatcher;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function save($has_trusted_data = FALSE) {
+    if (!$has_trusted_data) {
+      // @todo Use configuration schema to validate.
+      //   https://www.drupal.org/node/2270399
+      // Perform basic data validation.
+      foreach ($this->data as $key => $value) {
+        $this->validateValue($key, $value);
+      }
+    }
+
+    $this->storage->write($this->name, $this->data);
+    // Invalidate the cache tags not only when updating, but also when creating,
+    // because a language config override object uses the same cache tag as the
+    // default configuration object. Hence creating a language override is like
+    // an update of configuration, but only for a specific language.
+    Cache::invalidateTags($this->getCacheTags());
+    $this->isNew = FALSE;
+    // Dispatch configuration override event as detailed in
+    // \Drupal\Core\Config\ConfigFactoryOverrideInterface::createConfigObject().
+    $this->eventDispatcher->dispatch(new ConfigCrudEvent($this), ConfigCollectionEvents::SAVE_IN_COLLECTION);
+    // Dispatch an event specifically for language configuration override
+    // changes.
+    $this->eventDispatcher->dispatch(new LanguageConfigOverrideCrudEvent($this), LanguageConfigOverrideEvents::SAVE_OVERRIDE);
+    $this->originalData = $this->data;
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function delete() {
+    $this->data = [];
+    $this->storage->delete($this->name);
+    Cache::invalidateTags($this->getCacheTags());
+    $this->isNew = TRUE;
+    // Dispatch configuration override event as detailed in
+    // \Drupal\Core\Config\ConfigFactoryOverrideInterface::createConfigObject().
+    $this->eventDispatcher->dispatch(new ConfigCrudEvent($this), ConfigCollectionEvents::DELETE_IN_COLLECTION);
+    // Dispatch an event specifically for language configuration override
+    // changes.
+    $this->eventDispatcher->dispatch(new LanguageConfigOverrideCrudEvent($this), LanguageConfigOverrideEvents::DELETE_OVERRIDE);
+    $this->originalData = $this->data;
+    return $this;
+  }
+
+  /**
+   * Returns the language code of this language override.
+   *
+   * @return string
+   *   The language code.
+   */
+  public function getLangcode() {
+    return $this->getLangcodeFromCollectionName($this->getStorage()->getCollectionName());
+  }
+
+}

web/core/modules/language/src/Config/LanguageConfigOverrideCrudEvent.php

@@ -0,0 +1,41 @@
+<?php
+
+namespace Drupal\language\Config;
+
+use Drupal\Component\EventDispatcher\Event;
+
+/**
+ * Provides a language override event for event listeners.
+ *
+ * @see \Drupal\Core\Config\ConfigCrudEvent
+ */
+class LanguageConfigOverrideCrudEvent extends Event {
+
+  /**
+   * Configuration object.
+   *
+   * @var \Drupal\language\Config\LanguageConfigOverride
+   */
+  protected $override;
+
+  /**
+   * Constructs a configuration event object.
+   *
+   * @param \Drupal\language\Config\LanguageConfigOverride $override
+   *   Configuration object.
+   */
+  public function __construct(LanguageConfigOverride $override) {
+    $this->override = $override;
+  }
+
+  /**
+   * Gets configuration object.
+   *
+   * @return \Drupal\language\Config\LanguageConfigOverride
+   *   The configuration object that caused the event to fire.
+   */
+  public function getLanguageConfigOverride() {
+    return $this->override;
+  }
+
+}

web/core/modules/language/src/Config/LanguageConfigOverrideEvents.php

@@ -0,0 +1,42 @@
+<?php
+
+namespace Drupal\language\Config;
+
+/**
+ * Defines events for language configuration overrides.
+ *
+ * @see \Drupal\Core\Config\ConfigCrudEvent
+ */
+final class LanguageConfigOverrideEvents {
+
+  /**
+   * The name of the event fired when saving the configuration override.
+   *
+   * This event allows you to perform custom actions whenever a language config
+   * override is saved. The event listener method receives a
+   * \Drupal\language\Config\LanguageConfigOverrideCrudEvent instance.
+   *
+   * @Event
+   *
+   * @see \Drupal\language\Config\LanguageConfigOverrideCrudEvent
+   * @see \Drupal\language\Config\LanguageConfigOverride::save()
+   * @see \Drupal\locale\LocaleConfigSubscriber
+   */
+  const SAVE_OVERRIDE = 'language.save_override';
+
+  /**
+   * The name of the event fired when deleting the configuration override.
+   *
+   * This event allows you to perform custom actions whenever a language config
+   * override is deleted. The event listener method receives a
+   * \Drupal\language\Config\LanguageConfigOverrideCrudEvent instance.
+   *
+   * @Event
+   *
+   * @see \Drupal\language\Config\LanguageConfigOverrideCrudEvent
+   * @see \Drupal\language\Config\LanguageConfigOverride::delete()
+   * @see \Drupal\locale\LocaleConfigSubscriber
+   */
+  const DELETE_OVERRIDE = 'language.delete_override';
+
+}

web/core/modules/language/src/ConfigurableLanguageInterface.php

@@ -0,0 +1,34 @@
+<?php
+
+namespace Drupal\language;
+
+use Drupal\Core\Config\Entity\ConfigEntityInterface;
+use Drupal\Core\Language\LanguageInterface;
+
+/**
+ * Provides an interface defining a language entity.
+ */
+interface ConfigurableLanguageInterface extends ConfigEntityInterface, LanguageInterface {
+
+  /**
+   * Sets the name of the language.
+   *
+   * @param string $name
+   *   The human-readable English name of the language.
+   *
+   * @return $this
+   */
+  public function setName($name);
+
+  /**
+   * Sets the weight of the language.
+   *
+   * @param int $weight
+   *   The weight, used to order languages with larger positive weights sinking
+   *   items toward the bottom of lists.
+   *
+   * @return $this
+   */
+  public function setWeight($weight);
+
+}

web/core/modules/language/src/ConfigurableLanguageManager.php

@@ -0,0 +1,502 @@
+<?php
+
+namespace Drupal\language;
+
+use Drupal\Core\Language\LanguageInterface;
+use Drupal\Core\Config\ConfigFactoryInterface;
+use Drupal\Core\Extension\ModuleHandlerInterface;
+use Drupal\Core\Language\Language;
+use Drupal\Core\Language\LanguageDefault;
+use Drupal\Core\Language\LanguageManager;
+use Drupal\Core\StringTranslation\TranslatableMarkup;
+use Drupal\Core\Url;
+use Drupal\language\Config\LanguageConfigFactoryOverrideInterface;
+use Drupal\language\Entity\ConfigurableLanguage;
+use Symfony\Component\HttpFoundation\RequestStack;
+
+/**
+ * Overrides default LanguageManager to provide configured languages.
+ */
+class ConfigurableLanguageManager extends LanguageManager implements ConfigurableLanguageManagerInterface {
+
+  /**
+   * The configuration storage service.
+   *
+   * @var \Drupal\Core\Config\ConfigFactoryInterface
+   */
+  protected $configFactory;
+
+  /**
+   * The module handler service.
+   *
+   * @var \Drupal\Core\Extension\ModuleHandlerInterface
+   */
+  protected $moduleHandler;
+
+  /**
+   * The language configuration override service.
+   *
+   * @var \Drupal\language\Config\LanguageConfigFactoryOverrideInterface
+   */
+  protected $configFactoryOverride;
+
+  /**
+   * The request object.
+   *
+   * @var \Symfony\Component\HttpFoundation\RequestStack
+   */
+  protected $requestStack;
+
+  /**
+   * The language negotiator.
+   *
+   * @var \Drupal\language\LanguageNegotiatorInterface
+   */
+  protected $negotiator;
+
+  /**
+   * Local cache for language type configuration data.
+   *
+   * @var array
+   */
+  protected $languageTypes;
+
+  /**
+   * Local cache for language type information.
+   *
+   * @var array
+   */
+  protected $languageTypesInfo;
+
+  /**
+   * An array of language objects keyed by language type.
+   *
+   * @var \Drupal\Core\Language\LanguageInterface[]
+   */
+  protected $negotiatedLanguages;
+
+  /**
+   * An array of language negotiation method IDs keyed by language type.
+   *
+   * @var array
+   */
+  protected $negotiatedMethods;
+
+  /**
+   * Whether or not the language manager has been initialized.
+   *
+   * @var bool
+   */
+  protected $initialized = FALSE;
+
+  /**
+   * Whether language types are in the process of language initialization.
+   *
+   * @var bool[]
+   */
+  protected $initializing = [];
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function rebuildServices() {
+    \Drupal::service('kernel')->invalidateContainer();
+  }
+
+  /**
+   * Constructs a new ConfigurableLanguageManager object.
+   *
+   * @param \Drupal\Core\Language\LanguageDefault $default_language
+   *   The default language service.
+   * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
+   *   The configuration factory service.
+   * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
+   *   The module handler service.
+   * @param \Drupal\language\Config\LanguageConfigFactoryOverrideInterface $config_override
+   *   The language configuration override service.
+   * @param \Symfony\Component\HttpFoundation\RequestStack $request_stack
+   *   The request stack object.
+   */
+  public function __construct(LanguageDefault $default_language, ConfigFactoryInterface $config_factory, ModuleHandlerInterface $module_handler, LanguageConfigFactoryOverrideInterface $config_override, RequestStack $request_stack) {
+    $this->defaultLanguage = $default_language;
+    $this->configFactory = $config_factory;
+    $this->moduleHandler = $module_handler;
+    $this->configFactoryOverride = $config_override;
+    $this->requestStack = $request_stack;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function init() {
+    if (!$this->initialized) {
+      foreach ($this->getDefinedLanguageTypes() as $type) {
+        $this->getCurrentLanguage($type);
+      }
+      $this->initialized = TRUE;
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function isMultilingual() {
+    return count($this->getLanguages(LanguageInterface::STATE_CONFIGURABLE)) > 1;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getLanguageTypes() {
+    $this->loadLanguageTypesConfiguration();
+    return $this->languageTypes['configurable'];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getDefinedLanguageTypes() {
+    $this->loadLanguageTypesConfiguration();
+    return $this->languageTypes['all'];
+  }
+
+  /**
+   * Retrieves language types from the configuration storage.
+   *
+   * @return array
+   *   An array of language type names.
+   */
+  protected function loadLanguageTypesConfiguration() {
+    if (!$this->languageTypes) {
+      $this->languageTypes = $this->configFactory->get('language.types')->get() ?: ['configurable' => [], 'all' => parent::getLanguageTypes()];
+    }
+    return $this->languageTypes;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getDefinedLanguageTypesInfo() {
+    if (!isset($this->languageTypesInfo)) {
+      $defaults = parent::getDefinedLanguageTypesInfo();
+
+      $info = $this->moduleHandler->invokeAll('language_types_info');
+      $language_info = $info + $defaults;
+
+      // Let other modules alter the list of language types.
+      $this->moduleHandler->alter('language_types_info', $language_info);
+      $this->languageTypesInfo = $language_info;
+    }
+    return $this->languageTypesInfo;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function saveLanguageTypesConfiguration(array $values) {
+    $config = $this->configFactory->getEditable('language.types');
+    if (isset($values['configurable'])) {
+      $config->set('configurable', $values['configurable']);
+    }
+    if (isset($values['all'])) {
+      $config->set('all', $values['all']);
+    }
+    $config->save(TRUE);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getCurrentLanguage($type = LanguageInterface::TYPE_INTERFACE) {
+    if (!isset($this->negotiatedLanguages[$type])) {
+      // Ensure we have a valid value for this language type.
+      $this->negotiatedLanguages[$type] = $this->getDefaultLanguage();
+
+      if ($this->negotiator && $this->isMultilingual()) {
+        if (!isset($this->initializing[$type])) {
+          $this->initializing[$type] = TRUE;
+          $negotiation = $this->negotiator->initializeType($type);
+          $this->negotiatedLanguages[$type] = reset($negotiation);
+          $this->negotiatedMethods[$type] = key($negotiation);
+          unset($this->initializing[$type]);
+        }
+        // If the current interface language needs to be retrieved during
+        // initialization we return the system language. This way string
+        // translation calls happening during initialization will return the
+        // original strings which can be translated by calling them again
+        // afterwards. This can happen for instance while parsing negotiation
+        // method definitions.
+        elseif ($type == LanguageInterface::TYPE_INTERFACE) {
+          return new Language(['id' => LanguageInterface::LANGCODE_SYSTEM]);
+        }
+      }
+    }
+
+    return $this->negotiatedLanguages[$type];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function reset($type = NULL) {
+    if (!isset($type)) {
+      $this->initialized = FALSE;
+      $this->negotiatedLanguages = [];
+      $this->negotiatedMethods = [];
+      $this->languageTypes = NULL;
+      $this->languageTypesInfo = NULL;
+      $this->languages = [];
+      if ($this->negotiator) {
+        $this->negotiator->reset();
+      }
+    }
+    elseif (isset($this->negotiatedLanguages[$type])) {
+      unset($this->negotiatedLanguages[$type]);
+      unset($this->negotiatedMethods[$type]);
+    }
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getNegotiator() {
+    return $this->negotiator;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setNegotiator(LanguageNegotiatorInterface $negotiator) {
+    $this->negotiator = $negotiator;
+    $this->initialized = FALSE;
+    $this->negotiatedLanguages = [];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getLanguages($flags = LanguageInterface::STATE_CONFIGURABLE) {
+    // If a config override is set, cache using that language's ID.
+    if ($override_language = $this->getConfigOverrideLanguage()) {
+      $static_cache_id = $override_language->getId();
+    }
+    else {
+      $static_cache_id = $this->getCurrentLanguage()->getId();
+    }
+
+    if (!isset($this->languages[$static_cache_id][$flags])) {
+      // Initialize the language list with the default language and default
+      // locked languages. These cannot be removed. This serves as a fallback
+      // list if this method is invoked while the language module is installed
+      // and the configuration entities for languages are not yet fully
+      // imported.
+      $default = $this->getDefaultLanguage();
+      $languages = [$default->getId() => $default];
+      $languages += $this->getDefaultLockedLanguages($default->getWeight());
+
+      // Load configurable languages on top of the defaults. Ideally this could
+      // use the entity API to load and instantiate ConfigurableLanguage
+      // objects. However the entity API depends on the language system, so that
+      // would result in infinite loops. We use the configuration system
+      // directly and instantiate runtime Language objects. When language
+      // entities are imported those cover the default and locked languages, so
+      // site-specific configuration will prevail over the fallback values.
+      // Having them in the array already ensures if this is invoked in the
+      // middle of importing language configuration entities, the defaults are
+      // always present.
+      $config_ids = $this->configFactory->listAll('language.entity.');
+      foreach ($this->configFactory->loadMultiple($config_ids) as $config) {
+        $data = $config->get();
+        $data['name'] = $data['label'];
+        $languages[$data['id']] = new Language($data);
+      }
+      Language::sort($languages);
+
+      // Filter the full list of languages based on the value of $flags.
+      $this->languages[$static_cache_id][$flags] = $this->filterLanguages($languages, $flags);
+    }
+
+    return $this->languages[$static_cache_id][$flags];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getNativeLanguages() {
+    $languages = $this->getLanguages(LanguageInterface::STATE_CONFIGURABLE);
+    $natives = [];
+
+    $original_language = $this->getConfigOverrideLanguage();
+
+    foreach ($languages as $langcode => $language) {
+      $this->setConfigOverrideLanguage($language);
+      $natives[$langcode] = ConfigurableLanguage::load($langcode);
+    }
+    $this->setConfigOverrideLanguage($original_language);
+    Language::sort($natives);
+    return $natives;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function updateLockedLanguageWeights() {
+    // Get the weight of the last configurable language.
+    $configurable_languages = $this->getLanguages(LanguageInterface::STATE_CONFIGURABLE);
+    $max_weight = end($configurable_languages)->getWeight();
+
+    $locked_languages = $this->getLanguages(LanguageInterface::STATE_LOCKED);
+    // Update locked language weights to maintain the existing order, if
+    // necessary.
+    if (reset($locked_languages)->getWeight() <= $max_weight) {
+      foreach ($locked_languages as $language) {
+        // Update system languages weight.
+        $max_weight++;
+        ConfigurableLanguage::load($language->getId())
+          ->setWeight($max_weight)
+          ->save();
+      }
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFallbackCandidates(array $context = []) {
+    if ($this->isMultilingual()) {
+      $candidates = [];
+      if (empty($context['operation']) || $context['operation'] != 'locale_lookup') {
+        // If the fallback context is not locale_lookup, initialize the
+        // candidates with languages ordered by weight and add
+        // LanguageInterface::LANGCODE_NOT_SPECIFIED at the end. Interface
+        // translation fallback should only be based on explicit configuration
+        // gathered via the alter hooks below.
+        $candidates = array_keys($this->getLanguages());
+        $candidates[] = LanguageInterface::LANGCODE_NOT_SPECIFIED;
+        $candidates = array_combine($candidates, $candidates);
+
+        // The first candidate should always be the desired language if
+        // specified.
+        if (!empty($context['langcode'])) {
+          $candidates = [$context['langcode'] => $context['langcode']] + $candidates;
+        }
+      }
+
+      // Let other modules hook in and add/change candidates.
+      $type = 'language_fallback_candidates';
+      $types = [];
+      if (!empty($context['operation'])) {
+        $types[] = $type . '_' . $context['operation'];
+      }
+      $types[] = $type;
+      $this->moduleHandler->alter($types, $candidates, $context);
+    }
+    else {
+      $candidates = parent::getFallbackCandidates($context);
+    }
+
+    return $candidates;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getLanguageSwitchLinks($type, Url $url) {
+    if ($this->negotiator) {
+      foreach ($this->negotiator->getNegotiationMethods($type) as $method_id => $method) {
+        if (is_subclass_of($method['class'], LanguageSwitcherInterface::class)) {
+          $original_languages = $this->negotiatedLanguages;
+          $result = $this->negotiator->getNegotiationMethodInstance($method_id)->getLanguageSwitchLinks($this->requestStack->getCurrentRequest(), $type, $url);
+
+          if (!empty($result)) {
+            // Allow modules to provide translations for specific links.
+            $this->moduleHandler->alter('language_switch_links', $result, $type, $url);
+
+            $result = array_filter($result, function (array $link): bool {
+              $url = $link['url'] ?? NULL;
+              $language = $link['language'] ?? NULL;
+              if ($language instanceof LanguageInterface) {
+                $this->negotiatedLanguages[LanguageInterface::TYPE_CONTENT] = $language;
+                $this->negotiatedLanguages[LanguageInterface::TYPE_INTERFACE] = $language;
+              }
+              try {
+                return $url instanceof Url && $url->access();
+              }
+              catch (\Exception $e) {
+                return FALSE;
+              }
+            });
+            $this->negotiatedLanguages = $original_languages;
+
+            $links = (object) ['links' => $result, 'method_id' => $method_id];
+            break;
+          }
+        }
+      }
+    }
+
+    return $links ?? NULL;
+  }
+
+  /**
+   * Sets the configuration override language.
+   *
+   * @param \Drupal\Core\Language\LanguageInterface $language
+   *   The language to override configuration with.
+   *
+   * @return $this
+   */
+  public function setConfigOverrideLanguage(?LanguageInterface $language = NULL) {
+    $this->configFactoryOverride->setLanguage($language);
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getConfigOverrideLanguage() {
+    return $this->configFactoryOverride->getLanguage();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getLanguageConfigOverride($langcode, $name) {
+    return $this->configFactoryOverride->getOverride($langcode, $name);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getLanguageConfigOverrideStorage($langcode) {
+    return $this->configFactoryOverride->getStorage($langcode);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getStandardLanguageListWithoutConfigured() {
+    $languages = $this->getLanguages();
+    $predefined = $this->getStandardLanguageList();
+    foreach ($predefined as $key => $value) {
+      if (isset($languages[$key])) {
+        unset($predefined[$key]);
+        continue;
+      }
+      $predefined[$key] = new TranslatableMarkup($value[0]);
+    }
+    natcasesort($predefined);
+    return $predefined;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getNegotiatedLanguageMethod($type = LanguageInterface::TYPE_INTERFACE) {
+    if (isset($this->negotiatedLanguages[$type]) && isset($this->negotiatedMethods[$type])) {
+      return $this->negotiatedMethods[$type];
+    }
+  }
+
+}

web/core/modules/language/src/ConfigurableLanguageManagerInterface.php

@@ -0,0 +1,106 @@
+<?php
+
+namespace Drupal\language;
+
+use Drupal\Core\Language\LanguageInterface;
+use Drupal\Core\Language\LanguageManagerInterface;
+
+/**
+ * Common interface for language negotiation services.
+ */
+interface ConfigurableLanguageManagerInterface extends LanguageManagerInterface {
+
+  /**
+   * Rebuild the container to register services needed on multilingual sites.
+   */
+  public static function rebuildServices();
+
+  /**
+   * Returns the language negotiator.
+   *
+   * @return \Drupal\language\LanguageNegotiatorInterface
+   *   The language negotiator.
+   */
+  public function getNegotiator();
+
+  /**
+   * Injects the language negotiator.
+   *
+   * @param \Drupal\language\LanguageNegotiatorInterface $negotiator
+   *   The language negotiator.
+   */
+  public function setNegotiator(LanguageNegotiatorInterface $negotiator);
+
+  /**
+   * Returns all the defined language types including fixed ones.
+   *
+   * A language type maybe configurable or fixed. A fixed language type is a
+   * type whose language negotiation methods are module-defined and not altered
+   * through the user interface.
+   *
+   * @return array
+   *   An array of language type machine names.
+   */
+  public function getDefinedLanguageTypes();
+
+  /**
+   * Stores language types configuration.
+   *
+   * @param array $config
+   *   An indexed array with the following keys_
+   *   - configurable: an array of configurable language type names.
+   *   - all: an array of all the defined language type names.
+   */
+  public function saveLanguageTypesConfiguration(array $config);
+
+  /**
+   * Updates locked system language weights.
+   */
+  public function updateLockedLanguageWeights();
+
+  /**
+   * Gets a language config override object.
+   *
+   * @param string $langcode
+   *   The language code for the override.
+   * @param string $name
+   *   The language configuration object name.
+   *
+   * @return \Drupal\language\Config\LanguageConfigOverride
+   *   The language config override object.
+   */
+  public function getLanguageConfigOverride($langcode, $name);
+
+  /**
+   * Gets a language configuration override storage object.
+   *
+   * @param string $langcode
+   *   The language code for the override.
+   *
+   * @return \Drupal\Core\Config\StorageInterface
+   *   A storage object to use for reading and writing the
+   *   configuration override.
+   */
+  public function getLanguageConfigOverrideStorage($langcode);
+
+  /**
+   * Returns the standard language list excluding already configured languages.
+   *
+   * @return array
+   *   A list of standard language names keyed by langcode.
+   */
+  public function getStandardLanguageListWithoutConfigured();
+
+  /**
+   * Gets the negotiated language method ID.
+   *
+   * @param string $type
+   *   (optional) The language type; e.g., the interface or the content
+   *   language.
+   *
+   * @return string|null
+   *   The negotiated language method ID.
+   */
+  public function getNegotiatedLanguageMethod($type = LanguageInterface::TYPE_INTERFACE);
+
+}

web/core/modules/language/src/ContentLanguageSettingsException.php

@@ -0,0 +1,8 @@
+<?php
+
+namespace Drupal\language;
+
+/**
+ * Exception thrown by ContentLanguageSettings when target bundle is not set.
+ */
+class ContentLanguageSettingsException extends \RuntimeException {}

web/core/modules/language/src/ContentLanguageSettingsInterface.php

@@ -0,0 +1,78 @@
+<?php
+
+namespace Drupal\language;
+
+use Drupal\Core\Config\Entity\ConfigEntityInterface;
+
+/**
+ * Provides an interface defining language settings for content entities.
+ */
+interface ContentLanguageSettingsInterface extends ConfigEntityInterface {
+
+  /**
+   * Gets the entity type ID this config applies to.
+   *
+   * @return string
+   */
+  public function getTargetEntityTypeId();
+
+  /**
+   * Gets the bundle this config applies to.
+   *
+   * @return string
+   */
+  public function getTargetBundle();
+
+  /**
+   * Sets the bundle this config applies to.
+   *
+   * @param string $target_bundle
+   *   The bundle.
+   *
+   * @return $this
+   */
+  public function setTargetBundle($target_bundle);
+
+  /**
+   * Sets the default language code.
+   *
+   * @param string $default_langcode
+   *   The default language code.
+   *
+   * @return $this
+   */
+  public function setDefaultLangcode($default_langcode);
+
+  /**
+   * Gets the default language code.
+   *
+   * @return string
+   */
+  public function getDefaultLangcode();
+
+  /**
+   * Sets if the language must be alterable or not.
+   *
+   * @param bool $language_alterable
+   *   Flag indicating if the language must be alterable.
+   *
+   * @return $this
+   */
+  public function setLanguageAlterable($language_alterable);
+
+  /**
+   * Checks if the language is alterable or not.
+   *
+   * @return bool
+   */
+  public function isLanguageAlterable();
+
+  /**
+   * Checks if this config object contains the default values in every property.
+   *
+   * @return bool
+   *   True if all the properties contain the default values. False otherwise.
+   */
+  public function isDefaultConfiguration();
+
+}

web/core/modules/language/src/DefaultLanguageItem.php

@@ -0,0 +1,49 @@
+<?php
+
+namespace Drupal\language;
+
+use Drupal\Core\Entity\EntityInterface;
+use Drupal\Core\Field\Plugin\Field\FieldType\LanguageItem;
+use Drupal\Core\Language\Language;
+
+/**
+ * Alternative plugin implementation of the 'language' field type.
+ *
+ * Replaces the Core 'language' entity field type implementation, changes the
+ * default values used.
+ *
+ * Required settings are:
+ *  - target_type: The entity type to reference.
+ *
+ * @see language_field_info_alter().
+ */
+class DefaultLanguageItem extends LanguageItem {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function applyDefaultValue($notify = TRUE) {
+    // Default to LANGCODE_NOT_SPECIFIED.
+    $langcode = Language::LANGCODE_NOT_SPECIFIED;
+    if ($entity = $this->getEntity()) {
+      $langcode = $this->getDefaultLangcode($entity);
+    }
+    // Always notify otherwise default langcode will not be set correctly.
+    $this->setValue(['value' => $langcode], TRUE);
+    return $this;
+  }
+
+  /**
+   * Provides default language code of given entity.
+   *
+   * @param \Drupal\Core\Entity\EntityInterface $entity
+   *   The entity whose language code to be loaded.
+   *
+   * @return string
+   *   A string language code.
+   */
+  public function getDefaultLangcode(EntityInterface $entity) {
+    return language_get_default_langcode($entity->getEntityTypeId(), $entity->bundle());
+  }
+
+}

web/core/modules/language/src/Element/LanguageConfiguration.php

@@ -0,0 +1,116 @@
+<?php
+
+namespace Drupal\language\Element;
+
+use Drupal\Core\Url;
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Language\LanguageInterface;
+use Drupal\Core\Render\Attribute\FormElement;
+use Drupal\Core\Render\Element\FormElementBase;
+
+/**
+ * Defines an element for language configuration for a single field.
+ */
+#[FormElement('language_configuration')]
+class LanguageConfiguration extends FormElementBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getInfo() {
+    $class = static::class;
+    return [
+      '#input' => TRUE,
+      '#tree' => TRUE,
+      '#process' => [
+        [$class, 'processLanguageConfiguration'],
+      ],
+    ];
+  }
+
+  /**
+   * Process handler for the language_configuration form element.
+   */
+  public static function processLanguageConfiguration(&$element, FormStateInterface $form_state, &$form) {
+    $options = $element['#options'] ?? [];
+    // Avoid validation failure since we are moving the '#options' key in the
+    // nested 'language' select element.
+    unset($element['#options']);
+    /** @var \Drupal\language\Entity\ContentLanguageSettings $default_config */
+    $default_config = $element['#default_value'];
+    $element['langcode'] = [
+      '#type' => 'select',
+      '#title' => t('Default language'),
+      '#options' => $options + static::getDefaultOptions(),
+      '#description' => t('Explanation of the language options is found on the <a href=":languages_list_page">languages list page</a>.', [':languages_list_page' => Url::fromRoute('entity.configurable_language.collection')->toString()]),
+      '#default_value' => ($default_config != NULL) ? $default_config->getDefaultLangcode() : LanguageInterface::LANGCODE_SITE_DEFAULT,
+    ];
+
+    $element['language_alterable'] = [
+      '#type' => 'checkbox',
+      '#title' => t('Show language selector on create and edit pages'),
+      '#default_value' => ($default_config != NULL) ? $default_config->isLanguageAlterable() : FALSE,
+    ];
+
+    // Add the entity type and bundle information to the form if they are set.
+    // They will be used, in the submit handler, to generate the names of the
+    // configuration entities that will store the settings and are a way to uniquely
+    // identify the entity.
+    $language = $form_state->get('language') ?: [];
+    $language += [
+      $element['#name'] => [
+        'entity_type' => $element['#entity_information']['entity_type'],
+        'bundle' => $element['#entity_information']['bundle'],
+      ],
+    ];
+    $form_state->set('language', $language);
+
+    // Do not add the submit callback for the language content settings page,
+    // which is handled separately.
+    if ($form['#form_id'] != 'language_content_settings_form') {
+      // Determine where to attach the language_configuration element submit
+      // handler.
+      // @todo Form API: Allow form widgets/sections to declare #submit
+      //   handlers.
+      $submit_name = isset($form['actions']['save_continue']) ? 'save_continue' : 'submit';
+      if (isset($form['actions'][$submit_name]['#submit']) && array_search('language_configuration_element_submit', $form['actions'][$submit_name]['#submit']) === FALSE) {
+        $form['actions'][$submit_name]['#submit'][] = 'language_configuration_element_submit';
+      }
+      elseif (array_search('language_configuration_element_submit', $form['#submit']) === FALSE) {
+        $form['#submit'][] = 'language_configuration_element_submit';
+      }
+    }
+    return $element;
+  }
+
+  /**
+   * Returns the default options for the language configuration form element.
+   *
+   * @return array
+   *   An array containing the default options.
+   */
+  protected static function getDefaultOptions() {
+    $language_options = [
+      LanguageInterface::LANGCODE_SITE_DEFAULT => t("Site's default language (@language)", ['@language' => static::languageManager()->getDefaultLanguage()->getName()]),
+      'current_interface' => t('Interface text language selected for page'),
+      'authors_default' => t("Author's preferred language"),
+    ];
+
+    $languages = static::languageManager()->getLanguages(LanguageInterface::STATE_ALL);
+    foreach ($languages as $langcode => $language) {
+      $language_options[$langcode] = $language->isLocked() ? t('- @name -', ['@name' => $language->getName()]) : $language->getName();
+    }
+
+    return $language_options;
+  }
+
+  /**
+   * Wraps the language manager.
+   *
+   * @return \Drupal\Core\Language\LanguageManagerInterface
+   */
+  protected static function languageManager() {
+    return \Drupal::languageManager();
+  }
+
+}

web/core/modules/language/src/Entity/ConfigurableLanguage.php

@@ -0,0 +1,292 @@
+<?php
+
+namespace Drupal\language\Entity;
+
+use Drupal\Core\Config\Entity\ConfigEntityBase;
+use Drupal\Core\Entity\EntityStorageInterface;
+use Drupal\Core\Language\LanguageManager;
+use Drupal\language\ConfigurableLanguageManager;
+use Drupal\language\ConfigurableLanguageManagerInterface;
+use Drupal\language\Exception\DeleteDefaultLanguageException;
+use Drupal\language\ConfigurableLanguageInterface;
+
+/**
+ * Defines the ConfigurableLanguage entity.
+ *
+ * @ConfigEntityType(
+ *   id = "configurable_language",
+ *   label = @Translation("Language"),
+ *   label_collection = @Translation("Languages"),
+ *   label_singular = @Translation("language"),
+ *   label_plural = @Translation("languages"),
+ *   label_count = @PluralTranslation(
+ *     singular = "@count language",
+ *     plural = "@count languages",
+ *   ),
+ *   handlers = {
+ *     "list_builder" = "Drupal\language\LanguageListBuilder",
+ *     "access" = "Drupal\language\LanguageAccessControlHandler",
+ *     "form" = {
+ *       "add" = "Drupal\language\Form\LanguageAddForm",
+ *       "edit" = "Drupal\language\Form\LanguageEditForm",
+ *       "delete" = "Drupal\language\Form\LanguageDeleteForm"
+ *     }
+ *   },
+ *   admin_permission = "administer languages",
+ *   config_prefix = "entity",
+ *   entity_keys = {
+ *     "id" = "id",
+ *     "label" = "label",
+ *     "weight" = "weight"
+ *   },
+ *   config_export = {
+ *     "id",
+ *     "label",
+ *     "direction",
+ *     "weight",
+ *     "locked"
+ *   },
+ *   links = {
+ *     "delete-form" = "/admin/config/regional/language/delete/{configurable_language}",
+ *     "edit-form" = "/admin/config/regional/language/edit/{configurable_language}",
+ *     "collection" = "/admin/config/regional/language",
+ *   }
+ * )
+ */
+class ConfigurableLanguage extends ConfigEntityBase implements ConfigurableLanguageInterface {
+
+  /**
+   * The language ID (machine name).
+   *
+   * @var string
+   */
+  protected $id;
+
+  /**
+   * The human-readable label for the language.
+   *
+   * @var string
+   */
+  protected $label;
+
+  /**
+   * The direction of language, either DIRECTION_LTR or DIRECTION_RTL.
+   *
+   * @var int
+   */
+  protected $direction = self::DIRECTION_LTR;
+
+  /**
+   * The weight of the language, used in lists of languages.
+   *
+   * @var int
+   */
+  protected $weight = 0;
+
+  /**
+   * Locked languages cannot be edited.
+   *
+   * @var bool
+   */
+  protected $locked = FALSE;
+
+  /**
+   * Used during saving to detect when the site becomes multilingual.
+   *
+   * This property is not saved to the language entity, but is needed for
+   * detecting when to rebuild the services.
+   *
+   * @see \Drupal\language\Entity\ConfigurableLanguage::preSave()
+   * @see \Drupal\language\Entity\ConfigurableLanguage::postSave()
+   *
+   * @var bool
+   */
+  protected $preSaveMultilingual;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function isDefault() {
+    return static::getDefaultLangcode() == $this->id();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function isLocked() {
+    return (bool) $this->locked;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function preSave(EntityStorageInterface $storage) {
+    parent::preSave($storage);
+    // Store whether or not the site is already multilingual so that we can
+    // rebuild services if necessary during
+    // \Drupal\language\Entity\ConfigurableLanguage::postSave().
+    $this->preSaveMultilingual = \Drupal::languageManager()->isMultilingual();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function postSave(EntityStorageInterface $storage, $update = TRUE) {
+    parent::postSave($storage, $update);
+
+    $language_manager = \Drupal::languageManager();
+    $language_manager->reset();
+    if (!$this->isLocked() && $language_manager instanceof ConfigurableLanguageManagerInterface && !$this->isSyncing()) {
+      $language_manager->updateLockedLanguageWeights();
+    }
+
+    // Update URL Prefixes for all languages after the
+    // LanguageManagerInterface::getLanguages() cache is flushed.
+    language_negotiation_url_prefixes_update();
+
+    // If after adding this language the site will become multilingual, we need
+    // to rebuild language services.
+    if (!$this->preSaveMultilingual && !$update && $language_manager instanceof ConfigurableLanguageManagerInterface) {
+      $language_manager::rebuildServices();
+    }
+    if (!$update) {
+      // Install any available language configuration overrides for the language.
+      \Drupal::service('language.config_factory_override')->installLanguageOverrides($this->id());
+    }
+
+    if (!$this->isLocked() && !$update) {
+      // Add language to the list of language domains.
+      $config = \Drupal::configFactory()->getEditable('language.negotiation');
+      $domains = $config->get('url.domains');
+      $domains[$this->id()] = '';
+      $config->set('url.domains', $domains)->save(TRUE);
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   *
+   * @throws \Drupal\language\Exception\DeleteDefaultLanguageException
+   *   Exception thrown if we're trying to delete the default language entity.
+   *   This is not allowed as a site must have a default language.
+   */
+  public static function preDelete(EntityStorageInterface $storage, array $entities) {
+    $default_langcode = static::getDefaultLangcode();
+    foreach ($entities as $entity) {
+      if ($entity->id() == $default_langcode && !$entity->isUninstalling()) {
+        throw new DeleteDefaultLanguageException('Can not delete the default language');
+      }
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function postDelete(EntityStorageInterface $storage, array $entities) {
+    parent::postDelete($storage, $entities);
+    $language_manager = \Drupal::languageManager();
+    $language_manager->reset();
+    $entity = reset($entities);
+    if ($language_manager instanceof ConfigurableLanguageManagerInterface && !$entity->isUninstalling() && !$entity->isSyncing()) {
+      $language_manager->updateLockedLanguageWeights();
+    }
+    // If after deleting this language the site will become monolingual, we need
+    // to rebuild language services.
+    if (!\Drupal::languageManager()->isMultilingual()) {
+      ConfigurableLanguageManager::rebuildServices();
+    }
+
+    // Remove language from language prefix and domain list.
+    $config = \Drupal::configFactory()->getEditable('language.negotiation');
+    $config->clear('url.prefixes.' . $entity->id());
+    $config->clear('url.domains.' . $entity->id());
+    $config->save(TRUE);
+  }
+
+  /**
+   * Gets the default langcode.
+   *
+   * @return string
+   *   The current default langcode.
+   */
+  protected static function getDefaultLangcode() {
+    $language = \Drupal::service('language.default')->get();
+    return $language->getId();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getName() {
+    return $this->label();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setName($name) {
+    $this->label = $name;
+
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getId() {
+    return $this->id();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getDirection() {
+    return $this->direction;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getWeight() {
+    return $this->weight;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setWeight($weight) {
+    $this->weight = $weight;
+    return $this;
+  }
+
+  /**
+   * Creates a configurable language object from a langcode.
+   *
+   * @param string $langcode
+   *   The language code to use to create the object.
+   *
+   * @return $this
+   *
+   * @see \Drupal\Core\Language\LanguageManager::getStandardLanguageList()
+   */
+  public static function createFromLangcode($langcode) {
+    $standard_languages = LanguageManager::getStandardLanguageList();
+    if (!isset($standard_languages[$langcode])) {
+      // Drupal does not know about this language, so we set its values with the
+      // best guess. The user will be able to edit afterwards.
+      return static::create([
+        'id' => $langcode,
+        'label' => $langcode,
+      ]);
+    }
+    else {
+      // A known predefined language, details will be filled in properly.
+      return static::create([
+        'id' => $langcode,
+        'label' => $standard_languages[$langcode][0],
+        'direction' => $standard_languages[$langcode][2] ?? static::DIRECTION_LTR,
+      ]);
+    }
+  }
+
+}

web/core/modules/language/src/Entity/ContentLanguageSettings.php

@@ -0,0 +1,241 @@
+<?php
+
+namespace Drupal\language\Entity;
+
+use Drupal\Core\Config\Entity\ConfigEntityBase;
+use Drupal\Core\Entity\EntityStorageInterface;
+use Drupal\Core\Language\LanguageInterface;
+use Drupal\language\ContentLanguageSettingsException;
+use Drupal\language\ContentLanguageSettingsInterface;
+
+/**
+ * Defines the ContentLanguageSettings entity.
+ *
+ * @ConfigEntityType(
+ *   id = "language_content_settings",
+ *   label = @Translation("Content language settings"),
+ *   label_collection = @Translation("Content language settings"),
+ *   label_singular = @Translation("content language setting"),
+ *   label_plural = @Translation("content languages settings"),
+ *   label_count = @PluralTranslation(
+ *     singular = "@count content language setting",
+ *     plural = "@count content languages settings",
+ *   ),
+ *   admin_permission = "administer languages",
+ *   config_prefix = "content_settings",
+ *   entity_keys = {
+ *     "id" = "id"
+ *   },
+ *   config_export = {
+ *     "id",
+ *     "target_entity_type_id",
+ *     "target_bundle",
+ *     "default_langcode",
+ *     "language_alterable",
+ *   },
+ *   list_cache_tags = { "rendered" },
+ *   constraints = {
+ *     "ImmutableProperties" = {"id", "target_entity_type_id", "target_bundle"},
+ *   },
+ * )
+ */
+class ContentLanguageSettings extends ConfigEntityBase implements ContentLanguageSettingsInterface {
+
+  /**
+   * The id. Combination of $target_entity_type_id.$target_bundle.
+   *
+   * @var string
+   */
+  protected $id;
+
+  /**
+   * The entity type ID (machine name).
+   *
+   * @var string
+   */
+  protected $target_entity_type_id;
+
+  /**
+   * The bundle (machine name).
+   *
+   * @var string
+   */
+  protected $target_bundle;
+
+  /**
+   * The default language code.
+   *
+   * @var string
+   */
+  protected $default_langcode = LanguageInterface::LANGCODE_SITE_DEFAULT;
+
+  /**
+   * Indicates if the language is alterable or not.
+   *
+   * @var bool
+   */
+  protected $language_alterable = FALSE;
+
+  /**
+   * Constructs a ContentLanguageSettings object.
+   *
+   * In most cases, Field entities are created via
+   * FieldConfig::create($values), where $values is the same
+   * parameter as in this constructor.
+   *
+   * @param array $values
+   *   An array of the referring entity bundle with:
+   *   - target_entity_type_id: The entity type.
+   *   - target_bundle: The bundle.
+   *   Other array elements will be used to set the corresponding properties on
+   *   the class; see the class property documentation for details.
+   * @param string $entity_type
+   *   (optional) The entity type ID that is being created. Defaults to
+   *   "language_content_settings".
+   */
+  public function __construct(array $values, $entity_type = 'language_content_settings') {
+    if (empty($values['target_entity_type_id'])) {
+      throw new ContentLanguageSettingsException('Attempt to create content language settings without a target_entity_type_id.');
+    }
+    if (empty($values['target_bundle'])) {
+      throw new ContentLanguageSettingsException('Attempt to create content language settings without a target_bundle.');
+    }
+    parent::__construct($values, $entity_type);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function id() {
+    return $this->target_entity_type_id . '.' . $this->target_bundle;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getTargetEntityTypeId() {
+    return $this->target_entity_type_id;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getTargetBundle() {
+    return $this->target_bundle;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setTargetBundle($target_bundle) {
+    $this->target_bundle = $target_bundle;
+
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setDefaultLangcode($default_langcode) {
+    $this->default_langcode = $default_langcode;
+
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getDefaultLangcode() {
+    return $this->default_langcode;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setLanguageAlterable($language_alterable) {
+    $this->language_alterable = $language_alterable;
+
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function isLanguageAlterable() {
+    return $this->language_alterable;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function preSave(EntityStorageInterface $storage) {
+    $this->id = $this->id();
+    parent::preSave($storage);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function isDefaultConfiguration() {
+    return (!$this->language_alterable && $this->default_langcode == LanguageInterface::LANGCODE_SITE_DEFAULT);
+  }
+
+  /**
+   * Loads a content language config entity based on the entity type and bundle.
+   *
+   * @param string $entity_type_id
+   *   ID of the entity type.
+   * @param string $bundle
+   *   Bundle name.
+   *
+   * @return $this
+   *   The content language config entity if one exists. Otherwise, returns
+   *   default values.
+   */
+  public static function loadByEntityTypeBundle($entity_type_id, $bundle) {
+    if ($entity_type_id == NULL || $bundle == NULL) {
+      return NULL;
+    }
+    $config = \Drupal::entityTypeManager()->getStorage('language_content_settings')->load($entity_type_id . '.' . $bundle);
+    if ($config == NULL) {
+      $config = ContentLanguageSettings::create(['target_entity_type_id' => $entity_type_id, 'target_bundle' => $bundle]);
+    }
+    return $config;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function calculateDependencies() {
+    parent::calculateDependencies();
+
+    // Create dependency on the bundle.
+    $entity_type = \Drupal::entityTypeManager()->getDefinition($this->target_entity_type_id);
+    $bundle_config_dependency = $entity_type->getBundleConfigDependency($this->target_bundle);
+    $this->addDependency($bundle_config_dependency['type'], $bundle_config_dependency['name']);
+
+    return $this;
+  }
+
+  /**
+   * Returns all valid values for the `default_langcode` property.
+   *
+   * @return string[]
+   *   All possible valid default langcodes. This includes all langcodes in the
+   *   standard list of human languages, along with special langcodes like
+   *   `site_default`, `current_interface` and `authors_default`.
+   *
+   * @see \Drupal\language\Element\LanguageConfiguration::getDefaultOptions()
+   * @see \Drupal\Core\TypedData\Plugin\DataType\LanguageReference::getAllValidLangcodes()
+   */
+  public static function getAllValidDefaultLangcodes(): array {
+    $language_manager = \Drupal::service('language_manager');
+    return array_unique([
+      ...array_keys($language_manager->getLanguages(LanguageInterface::STATE_ALL)),
+      LanguageInterface::LANGCODE_SITE_DEFAULT,
+      'current_interface',
+      'authors_default',
+    ]);
+  }
+
+}

web/core/modules/language/src/EventSubscriber/ConfigSubscriber.php

@@ -0,0 +1,148 @@
+<?php
+
+namespace Drupal\language\EventSubscriber;
+
+use Drupal\Core\Config\ConfigFactoryInterface;
+use Drupal\Core\Language\Language;
+use Drupal\Core\Language\LanguageDefault;
+use Drupal\Core\Language\LanguageManagerInterface;
+use Drupal\Core\Config\ConfigCrudEvent;
+use Drupal\Core\Config\ConfigEvents;
+use Drupal\language\ConfigurableLanguageManager;
+use Drupal\language\HttpKernel\PathProcessorLanguage;
+use Drupal\language\LanguageNegotiatorInterface;
+use Symfony\Component\EventDispatcher\EventSubscriberInterface;
+
+/**
+ * Deletes the container if default language has changed.
+ */
+class ConfigSubscriber implements EventSubscriberInterface {
+
+  /**
+   * The language manager.
+   *
+   * @var \Drupal\Core\Language\LanguageManagerInterface
+   */
+  protected $languageManager;
+
+  /**
+   * The default language.
+   *
+   * @var \Drupal\Core\Language\LanguageDefault
+   */
+  protected $languageDefault;
+
+  /**
+   * The configuration factory.
+   *
+   * @var \Drupal\Core\Config\ConfigFactoryInterface
+   */
+  protected $configFactory;
+
+  /**
+   * The language negotiator.
+   *
+   * @var \Drupal\language\LanguageNegotiatorInterface
+   */
+  protected $languageNegotiator;
+
+  /**
+   * The language path processor.
+   *
+   * @var \Drupal\language\HttpKernel\PathProcessorLanguage
+   */
+  protected $pathProcessorLanguage;
+
+  /**
+   * Constructs a new class object.
+   *
+   * @param \Drupal\Core\Language\LanguageManagerInterface $language_manager
+   *   The language manager.
+   * @param \Drupal\Core\Language\LanguageDefault $language_default
+   *   The default language.
+   * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
+   *   The configuration factory.
+   * @param \Drupal\language\LanguageNegotiatorInterface $language_negotiator
+   *   The language negotiator.
+   */
+  public function __construct(LanguageManagerInterface $language_manager, LanguageDefault $language_default, ConfigFactoryInterface $config_factory, LanguageNegotiatorInterface $language_negotiator) {
+    $this->languageManager = $language_manager;
+    $this->languageDefault = $language_default;
+    $this->configFactory = $config_factory;
+    $this->languageNegotiator = $language_negotiator;
+  }
+
+  /**
+   * Causes the container to be rebuilt on the next request.
+   *
+   * This event subscriber assumes that the new default langcode and old default
+   * langcode are valid langcodes. If the schema definition of either
+   * system.site:default_langcode or language.negotiation::url.prefixes changes
+   * then this event must be changed to work with both the old and new schema
+   * definition so this event is update safe.
+   *
+   * @param \Drupal\Core\Config\ConfigCrudEvent $event
+   *   The configuration event.
+   */
+  public function onConfigSave(ConfigCrudEvent $event) {
+    $saved_config = $event->getConfig();
+    if ($saved_config->getName() == 'system.site' && $event->isChanged('default_langcode')) {
+      $new_default_langcode = $saved_config->get('default_langcode');
+      $default_language = $this->configFactory->get('language.entity.' . $new_default_langcode);
+      // During an import the language might not exist yet.
+      if (!$default_language->isNew()) {
+        $this->languageDefault->set(new Language($default_language->get()));
+        $this->languageManager->reset();
+
+        // Directly update language negotiation settings instead of calling
+        // language_negotiation_url_prefixes_update() to ensure that the code
+        // obeys the hook_update_N() restrictions.
+        $negotiation_config = $this->configFactory->getEditable('language.negotiation');
+        $negotiation_changed = FALSE;
+        $url_prefixes = $negotiation_config->get('url.prefixes');
+        $old_default_langcode = $saved_config->getOriginal('default_langcode');
+        if (empty($url_prefixes[$old_default_langcode])) {
+          $negotiation_config->set('url.prefixes.' . $old_default_langcode, $old_default_langcode);
+          $negotiation_changed = TRUE;
+        }
+        if (empty($url_prefixes[$new_default_langcode])) {
+          $negotiation_config->set('url.prefixes.' . $new_default_langcode, '');
+          $negotiation_changed = TRUE;
+        }
+        if ($negotiation_changed) {
+          $negotiation_config->save(TRUE);
+        }
+      }
+      // Trigger a container rebuild on the next request by invalidating it.
+      ConfigurableLanguageManager::rebuildServices();
+    }
+    elseif ($saved_config->getName() == 'language.types' && $event->isChanged('negotiation')) {
+      // If the negotiation configuration changed the language negotiator and
+      // the language path processor have to be reset so that they regenerate
+      // the method instances and also sort them accordingly to the new config.
+      $this->languageNegotiator->reset();
+      if (isset($this->pathProcessorLanguage)) {
+        $this->pathProcessorLanguage->reset();
+      }
+    }
+  }
+
+  /**
+   * Injects the language path processors on multilingual site configuration.
+   *
+   * @param \Drupal\language\HttpKernel\PathProcessorLanguage $path_processor_language
+   *   The language path processor.
+   */
+  public function setPathProcessorLanguage(PathProcessorLanguage $path_processor_language) {
+    $this->pathProcessorLanguage = $path_processor_language;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function getSubscribedEvents(): array {
+    $events[ConfigEvents::SAVE][] = ['onConfigSave', 0];
+    return $events;
+  }
+
+}

web/core/modules/language/src/EventSubscriber/LanguageRequestSubscriber.php

@@ -0,0 +1,113 @@
+<?php
+
+namespace Drupal\language\EventSubscriber;
+
+use Drupal\Core\DrupalKernelInterface;
+use Drupal\Core\Session\AccountInterface;
+use Drupal\Core\StringTranslation\Translator\TranslatorInterface;
+use Drupal\language\ConfigurableLanguageManagerInterface;
+use Drupal\language\LanguageNegotiatorInterface;
+use Symfony\Component\HttpKernel\KernelEvents;
+use Symfony\Component\HttpKernel\Event\RequestEvent;
+use Symfony\Component\EventDispatcher\EventSubscriberInterface;
+
+/**
+ * Sets the $request property on the language manager.
+ */
+class LanguageRequestSubscriber implements EventSubscriberInterface {
+
+  /**
+   * The language manager service.
+   *
+   * @var \Drupal\language\ConfigurableLanguageManagerInterface
+   */
+  protected $languageManager;
+
+  /**
+   * The language negotiator.
+   *
+   * @var \Drupal\language\LanguageNegotiatorInterface
+   */
+  protected $negotiator;
+
+  /**
+   * The translation service.
+   *
+   * @var \Drupal\Core\StringTranslation\Translator\TranslatorInterface
+   */
+  protected $translation;
+
+  /**
+   * The current active user.
+   *
+   * @var \Drupal\Core\Session\AccountInterface
+   */
+  protected $currentUser;
+
+  /**
+   * Constructs a LanguageRequestSubscriber object.
+   *
+   * @param \Drupal\language\ConfigurableLanguageManagerInterface $language_manager
+   *   The language manager service.
+   * @param \Drupal\language\LanguageNegotiatorInterface $negotiator
+   *   The language negotiator.
+   * @param \Drupal\Core\StringTranslation\Translator\TranslatorInterface $translation
+   *   The translation service.
+   * @param \Drupal\Core\Session\AccountInterface $current_user
+   *   The current active user.
+   */
+  public function __construct(ConfigurableLanguageManagerInterface $language_manager, LanguageNegotiatorInterface $negotiator, TranslatorInterface $translation, AccountInterface $current_user) {
+    $this->languageManager = $language_manager;
+    $this->negotiator = $negotiator;
+    $this->translation = $translation;
+    $this->currentUser = $current_user;
+  }
+
+  /**
+   * Initializes the language manager at the beginning of the request.
+   *
+   * @param \Symfony\Component\HttpKernel\Event\RequestEvent $event
+   *   The Event to process.
+   */
+  public function onKernelRequestLanguage(RequestEvent $event) {
+    if ($event->isMainRequest()) {
+      $this->setLanguageOverrides();
+    }
+  }
+
+  /**
+   * Initializes config overrides whenever the service container is rebuilt.
+   */
+  public function onContainerInitializeSubrequestFinished() {
+    $this->setLanguageOverrides();
+  }
+
+  /**
+   * Sets the language for config overrides on the language manager.
+   */
+  private function setLanguageOverrides() {
+    $this->negotiator->setCurrentUser($this->currentUser);
+    if ($this->languageManager instanceof ConfigurableLanguageManagerInterface) {
+      $this->languageManager->setNegotiator($this->negotiator);
+      $this->languageManager->setConfigOverrideLanguage($this->languageManager->getCurrentLanguage());
+    }
+    // After the language manager has initialized, set the default langcode for
+    // the string translations.
+    $langcode = $this->languageManager->getCurrentLanguage()->getId();
+    $this->translation->setDefaultLangcode($langcode);
+  }
+
+  /**
+   * Registers the methods in this class that should be listeners.
+   *
+   * @return array
+   *   An array of event listener definitions.
+   */
+  public static function getSubscribedEvents(): array {
+    $events[KernelEvents::REQUEST][] = ['onKernelRequestLanguage', 255];
+    $events[DrupalKernelInterface::CONTAINER_INITIALIZE_SUBREQUEST_FINISHED][] = ['onContainerInitializeSubrequestFinished', 255];
+
+    return $events;
+  }
+
+}

web/core/modules/language/src/Exception/DeleteDefaultLanguageException.php

@@ -0,0 +1,8 @@
+<?php
+
+namespace Drupal\language\Exception;
+
+/**
+ * Exception thrown when deleting the default language.
+ */
+class DeleteDefaultLanguageException extends LanguageException {}

web/core/modules/language/src/Exception/LanguageException.php

@@ -0,0 +1,8 @@
+<?php
+
+namespace Drupal\language\Exception;
+
+/**
+ * A base exception thrown in any language system operations.
+ */
+class LanguageException extends \RuntimeException {}

web/core/modules/language/src/Form/ContentLanguageSettingsForm.php

@@ -0,0 +1,209 @@
+<?php
+
+namespace Drupal\language\Form;
+
+use Drupal\Core\Entity\ContentEntityTypeInterface;
+use Drupal\Core\Entity\EntityTypeBundleInfoInterface;
+use Drupal\Core\Entity\EntityTypeManagerInterface;
+use Drupal\Core\Extension\ModuleHandlerInterface;
+use Drupal\Core\Form\FormBase;
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\language\Entity\ContentLanguageSettings;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Configure the content language settings for this site.
+ *
+ * @internal
+ */
+class ContentLanguageSettingsForm extends FormBase {
+
+  /**
+   * The entity type manager.
+   *
+   * @var \Drupal\Core\Entity\EntityTypeManagerInterface
+   */
+  protected $entityTypeManager;
+
+  /**
+   * The entity bundle info.
+   *
+   * @var \Drupal\Core\Entity\EntityTypeBundleInfoInterface
+   */
+  protected $entityTypeBundleInfo;
+
+  /**
+   * If this validator can handle multiple arguments.
+   *
+   * @var bool
+   */
+  protected $multipleCapable = TRUE;
+
+  /**
+   * The module handler.
+   *
+   * @var \Drupal\Core\Extension\ModuleHandlerInterface
+   */
+  protected $moduleHandler;
+
+  /**
+   * Constructs an \Drupal\views\Plugin\views\argument_validator\Entity object.
+   *
+   * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager
+   *   The entity type manager.
+   * @param \Drupal\Core\Entity\EntityTypeBundleInfoInterface $entity_type_bundle_info
+   *   The entity type bundle info.
+   * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
+   *   The module handler.
+   */
+  public function __construct(EntityTypeManagerInterface $entity_type_manager, EntityTypeBundleInfoInterface $entity_type_bundle_info, ModuleHandlerInterface $module_handler) {
+    $this->entityTypeManager = $entity_type_manager;
+    $this->entityTypeBundleInfo = $entity_type_bundle_info;
+    $this->moduleHandler = $module_handler;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container) {
+    return new static(
+      $container->get('entity_type.manager'),
+      $container->get('entity_type.bundle.info'),
+      $container->get('module_handler')
+    );
+  }
+
+  /**
+   * The _title_callback for the language.content_settings_page route.
+   *
+   * @return \Drupal\Core\StringTranslation\TranslatableMarkup
+   *   The page title.
+   */
+  public function getTitle(): string {
+    if ($this->moduleHandler->moduleExists('content_translation')) {
+      return $this->t('Content language and translation');
+    }
+    else {
+      return $this->t('Content language');
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFormId() {
+    return 'language_content_settings_form';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildForm(array $form, FormStateInterface $form_state) {
+    $entity_types = $this->entityTypeManager->getDefinitions();
+    $labels = [];
+    $default = [];
+
+    $bundles = $this->entityTypeBundleInfo->getAllBundleInfo();
+    $language_configuration = [];
+    foreach ($entity_types as $entity_type_id => $entity_type) {
+      if (!$entity_type instanceof ContentEntityTypeInterface || !$entity_type->hasKey('langcode') || !isset($bundles[$entity_type_id])) {
+        continue;
+      }
+      $labels[$entity_type_id] = $entity_type->getLabel() ?: $entity_type_id;
+      $default[$entity_type_id] = FALSE;
+
+      // Check whether we have any custom setting.
+      foreach ($bundles[$entity_type_id] as $bundle => $bundle_info) {
+        $config = ContentLanguageSettings::loadByEntityTypeBundle($entity_type_id, $bundle);
+        if (!$config->isDefaultConfiguration()) {
+          $default[$entity_type_id] = $entity_type_id;
+        }
+        $language_configuration[$entity_type_id][$bundle] = $config;
+      }
+    }
+
+    asort($labels);
+
+    $form = [
+      '#labels' => $labels,
+      '#attached' => [
+        'library' => [
+          'language/drupal.language.admin',
+        ],
+      ],
+      '#attributes' => [
+        'class' => 'language-content-settings-form',
+      ],
+    ];
+
+    $form['entity_types'] = [
+      '#title' => $this->t('Custom language settings'),
+      '#type' => 'checkboxes',
+      '#options' => $labels,
+      '#default_value' => $default,
+    ];
+
+    $form['settings'] = ['#tree' => TRUE];
+
+    foreach ($labels as $entity_type_id => $label) {
+      $entity_type = $entity_types[$entity_type_id];
+
+      $form['settings'][$entity_type_id] = [
+        '#title' => $label,
+        '#type' => 'details',
+        '#entity_type' => $entity_type_id,
+        '#theme' => 'language_content_settings_table',
+        '#bundle_label' => $entity_type->getBundleLabel() ?: $label,
+        '#states' => [
+          'visible' => [
+            ':input[name="entity_types[' . $entity_type_id . ']"]' => ['checked' => TRUE],
+          ],
+        ],
+      ];
+
+      foreach ($bundles[$entity_type_id] as $bundle => $bundle_info) {
+        $form['settings'][$entity_type_id][$bundle]['settings'] = [
+          '#type' => 'item',
+          '#label' => $bundle_info['label'],
+          'language' => [
+            '#type' => 'language_configuration',
+            '#entity_information' => [
+              'entity_type' => $entity_type_id,
+              'bundle' => $bundle,
+            ],
+            '#default_value' => $language_configuration[$entity_type_id][$bundle],
+          ],
+        ];
+      }
+    }
+
+    $form['actions']['#type'] = 'actions';
+    $form['actions']['submit'] = [
+      '#type' => 'submit',
+      '#value' => $this->t('Save configuration'),
+      '#button_type' => 'primary',
+    ];
+
+    return $form;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function submitForm(array &$form, FormStateInterface $form_state) {
+    $entity_types = $form_state->getValue('entity_types');
+    foreach ($form_state->getValue('settings') as $entity_type => $entity_settings) {
+      foreach ($entity_settings as $bundle => $bundle_settings) {
+        $config = ContentLanguageSettings::loadByEntityTypeBundle($entity_type, $bundle);
+        if (empty($entity_types[$entity_type])) {
+          $bundle_settings['settings']['language']['language_alterable'] = FALSE;
+        }
+        $config->setDefaultLangcode($bundle_settings['settings']['language']['langcode'])
+          ->setLanguageAlterable($bundle_settings['settings']['language']['language_alterable'])
+          ->save();
+      }
+    }
+    $this->messenger()->addStatus($this->t('Settings successfully updated.'));
+  }
+
+}

web/core/modules/language/src/Form/LanguageAddForm.php

@@ -0,0 +1,168 @@
+<?php
+
+namespace Drupal\language\Form;
+
+use Drupal\Core\Entity\EntityInterface;
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Language\LanguageManager;
+use Drupal\Core\Url;
+use Drupal\language\Entity\ConfigurableLanguage;
+
+/**
+ * Controller for language addition forms.
+ *
+ * @internal
+ */
+class LanguageAddForm extends LanguageFormBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFormId() {
+    // @todo Remove in favor of base method.
+    return 'language_admin_add_form';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function form(array $form, FormStateInterface $form_state) {
+    $form['#title'] = $this->t('Add language');
+
+    $predefined_languages = $this->languageManager->getStandardLanguageListWithoutConfigured();
+
+    $predefined_languages['custom'] = $this->t('Custom language...');
+    $predefined_default = $form_state->getValue('predefined_langcode', key($predefined_languages));
+    $form['predefined_langcode'] = [
+      '#type' => 'select',
+      '#title' => $this->t('Language name'),
+      '#default_value' => $predefined_default,
+      '#options' => $predefined_languages,
+    ];
+    $form['predefined_submit'] = [
+      '#type' => 'submit',
+      '#value' => $this->t('Add language'),
+      '#name' => 'add_language',
+      '#limit_validation_errors' => [['predefined_langcode'], ['predefined_submit']],
+      '#states' => [
+        'invisible' => [
+          'select#edit-predefined-langcode' => ['value' => 'custom'],
+        ],
+      ],
+      '#validate' => ['::validatePredefined'],
+      '#submit' => ['::submitForm', '::save'],
+      '#button_type' => 'primary',
+    ];
+
+    $custom_language_states_conditions = [
+      'select#edit-predefined-langcode' => ['value' => 'custom'],
+    ];
+    $form['custom_language'] = [
+      '#type' => 'container',
+      '#states' => [
+        'visible' => $custom_language_states_conditions,
+      ],
+    ];
+    $this->commonForm($form['custom_language']);
+    $form['custom_language']['langcode']['#states'] = [
+      'required' => $custom_language_states_conditions,
+    ];
+    $form['custom_language']['label']['#states'] = [
+      'required' => $custom_language_states_conditions,
+    ];
+    $form['custom_language']['submit'] = [
+      '#type' => 'submit',
+      '#value' => $this->t('Add custom language'),
+      '#name' => 'add_custom_language',
+      '#validate' => ['::validateCustom'],
+      '#submit' => ['::submitForm', '::save'],
+    ];
+
+    return $form;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function save(array $form, FormStateInterface $form_state) {
+    parent::save($form, $form_state);
+
+    $t_args = ['%language' => $this->entity->label(), '%langcode' => $this->entity->id()];
+    $this->logger('language')->notice('The %language (%langcode) language has been created.', $t_args);
+    $this->messenger()->addStatus($this->t('The language %language has been created and can now be used.', $t_args));
+
+    if ($this->moduleHandler->moduleExists('block')) {
+      // Tell the user they have the option to add a language switcher block
+      // to their theme so they can switch between the languages.
+      $this->messenger()->addStatus($this->t('Use one of the language switcher blocks to allow site visitors to switch between languages. You can enable these blocks on the <a href=":block-admin">block administration page</a>.', [':block-admin' => Url::fromRoute('block.admin_display')->toString()]));
+    }
+    $form_state->setRedirectUrl($this->entity->toUrl('collection'));
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function actions(array $form, FormStateInterface $form_state) {
+    // No actions needed.
+    return [];
+  }
+
+  /**
+   * Validates the language addition form on custom language button.
+   */
+  public function validateCustom(array $form, FormStateInterface $form_state) {
+    if ($form_state->getValue('predefined_langcode') == 'custom') {
+      $langcode = $form_state->getValue('langcode');
+      // Reuse the editing form validation routine if we add a custom language.
+      $this->validateCommon($form['custom_language'], $form_state);
+
+      if ($language = $this->languageManager->getLanguage($langcode)) {
+        $form_state->setErrorByName('langcode', $this->t('The language %language (%langcode) already exists.', ['%language' => $language->getName(), '%langcode' => $langcode]));
+      }
+    }
+    else {
+      $form_state->setErrorByName('predefined_langcode', $this->t('Use the <em>Add language</em> button to save a predefined language.'));
+    }
+  }
+
+  /**
+   * Element specific validator for the Add language button.
+   */
+  public function validatePredefined($form, FormStateInterface $form_state) {
+    $langcode = $form_state->getValue('predefined_langcode');
+    if ($langcode == 'custom') {
+      $form_state->setErrorByName('predefined_langcode', $this->t('Fill in the language details and save the language with <em>Add custom language</em>.'));
+    }
+    else {
+      if ($language = $this->languageManager->getLanguage($langcode)) {
+        $form_state->setErrorByName('predefined_langcode', $this->t('The language %language (%langcode) already exists.', ['%language' => $language->getName(), '%langcode' => $langcode]));
+      }
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function copyFormValuesToEntity(EntityInterface $entity, array $form, FormStateInterface $form_state) {
+    $langcode = $form_state->getValue('predefined_langcode');
+    if ($langcode == 'custom') {
+      $langcode = $form_state->getValue('langcode');
+      $label = $form_state->getValue('label');
+      $direction = $form_state->getValue('direction');
+    }
+    else {
+      $standard_languages = LanguageManager::getStandardLanguageList();
+      $label = $standard_languages[$langcode][0];
+      $direction = $standard_languages[$langcode][2] ?? ConfigurableLanguage::DIRECTION_LTR;
+    }
+    $entity->set('id', $langcode);
+    $entity->set('label', $label);
+    $entity->set('direction', $direction);
+    // There is no weight on the edit form. Fetch all configurable languages
+    // ordered by weight and set the new language to be placed after them.
+    $languages = \Drupal::languageManager()->getLanguages(ConfigurableLanguage::STATE_CONFIGURABLE);
+    $last_language = end($languages);
+    $entity->setWeight($last_language->getWeight() + 1);
+  }
+
+}

web/core/modules/language/src/Form/LanguageDeleteForm.php

@@ -0,0 +1,42 @@
+<?php
+
+namespace Drupal\language\Form;
+
+use Drupal\Core\Entity\EntityDeleteForm;
+
+/**
+ * Defines a confirmation form for deleting a language entity.
+ *
+ * @internal
+ */
+class LanguageDeleteForm extends EntityDeleteForm {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getDescription() {
+    return $this->t('Deleting a language will remove all interface translations associated with it, and content in this language will be set to be language neutral. This action cannot be undone.');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFormId() {
+    return 'language_delete_form';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function getDeletionMessage() {
+    return $this->t('The %language (%langcode) language has been removed.', ['%language' => $this->entity->label(), '%langcode' => $this->entity->id()]);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function logDeletionMessage() {
+    $this->logger('language')->notice('The %language (%langcode) language has been removed.', ['%language' => $this->entity->label(), '%langcode' => $this->entity->id()]);
+  }
+
+}

web/core/modules/language/src/Form/LanguageEditForm.php

@@ -0,0 +1,52 @@
+<?php
+
+namespace Drupal\language\Form;
+
+use Drupal\Core\Form\FormStateInterface;
+
+/**
+ * Controller for language edit forms.
+ *
+ * @internal
+ */
+class LanguageEditForm extends LanguageFormBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFormId() {
+    // @todo Remove in favor of base method.
+    return 'language_admin_edit_form';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function form(array $form, FormStateInterface $form_state) {
+    $this->commonForm($form);
+    return parent::form($form, $form_state);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function actions(array $form, FormStateInterface $form_state) {
+    $actions['submit'] = [
+      '#type' => 'submit',
+      '#value' => $this->t('Save language'),
+      '#validate' => ['::validateCommon'],
+      '#submit' => ['::submitForm', '::save'],
+    ];
+    return $actions;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function save(array $form, FormStateInterface $form_state) {
+    parent::save($form, $form_state);
+    $form_state->setRedirectUrl($this->entity->toUrl('collection'));
+    $this->logger('language')->notice('The %language (%langcode) language has been updated.', ['%language' => $this->entity->label(), '%langcode' => $this->entity->id()]);
+  }
+
+}

web/core/modules/language/src/Form/LanguageFormBase.php

@@ -0,0 +1,109 @@
+<?php
+
+namespace Drupal\language\Form;
+
+use Drupal\Component\Utility\Html;
+use Drupal\Core\Entity\EntityForm;
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Language\LanguageInterface;
+use Drupal\language\ConfigurableLanguageManagerInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Base form for language add and edit forms.
+ */
+abstract class LanguageFormBase extends EntityForm {
+
+  /**
+   * The configurable language manager.
+   *
+   * @var \Drupal\language\ConfigurableLanguageManagerInterface
+   */
+  protected $languageManager;
+
+  /**
+   * Constructs a ContentEntityForm object.
+   *
+   * @param \Drupal\language\ConfigurableLanguageManagerInterface $language_manager
+   *   The configurable language manager.
+   */
+  public function __construct(ConfigurableLanguageManagerInterface $language_manager) {
+    $this->languageManager = $language_manager;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container) {
+    return new static(
+      $container->get('language_manager')
+    );
+  }
+
+  /**
+   * Common elements of the language addition and editing form.
+   */
+  public function commonForm(array &$form) {
+    /** @var \Drupal\language\ConfigurableLanguageInterface $language */
+    $language = $this->entity;
+    if ($language->getId()) {
+      $form['langcode_view'] = [
+        '#type' => 'item',
+        '#title' => $this->t('Language code'),
+        '#markup' => $language->id(),
+      ];
+      $form['langcode'] = [
+        '#type' => 'value',
+        '#value' => $language->id(),
+      ];
+    }
+    else {
+      $form['langcode'] = [
+        '#type' => 'textfield',
+        '#title' => $this->t('Language code'),
+        '#maxlength' => 12,
+        '#required' => TRUE,
+        '#default_value' => '',
+        '#disabled' => FALSE,
+        '#description' => $this->t('Use language codes as <a href=":w3ctags">defined by the W3C</a> for interoperability. <em>Examples: "en", "en-gb" and "zh-hant".</em>', [':w3ctags' => 'http://www.w3.org/International/articles/language-tags/']),
+      ];
+    }
+    $form['label'] = [
+      '#type' => 'textfield',
+      '#title' => $this->t('Language name'),
+      '#maxlength' => 64,
+      '#default_value' => $language->label(),
+      '#required' => TRUE,
+    ];
+    $form['direction'] = [
+      '#type' => 'radios',
+      '#title' => $this->t('Direction'),
+      '#required' => TRUE,
+      '#description' => $this->t('Direction that text in this language is presented.'),
+      '#default_value' => $language->getDirection(),
+      '#options' => [
+        LanguageInterface::DIRECTION_LTR => $this->t('Left to right'),
+        LanguageInterface::DIRECTION_RTL => $this->t('Right to left'),
+      ],
+    ];
+
+    return $form;
+  }
+
+  /**
+   * Validates the language editing element.
+   */
+  public function validateCommon(array $form, FormStateInterface $form_state) {
+    // Ensure sane field values for langcode and name.
+    if (!isset($form['langcode_view']) && !preg_match('@^' . LanguageInterface::VALID_LANGCODE_REGEX . '$@', $form_state->getValue('langcode'))) {
+      $form_state->setErrorByName('langcode', $this->t('%field must be a valid language tag as <a href=":url">defined by the W3C</a>.', [
+        '%field' => $form['langcode']['#title'],
+        ':url' => 'http://www.w3.org/International/articles/language-tags/',
+      ]));
+    }
+    if ($form_state->getValue('label') != Html::escape($form_state->getValue('label'))) {
+      $form_state->setErrorByName('label', $this->t('%field cannot contain any markup.', ['%field' => $form['label']['#title']]));
+    }
+  }
+
+}

web/core/modules/language/src/Form/NegotiationBrowserDeleteForm.php

@@ -0,0 +1,83 @@
+<?php
+
+namespace Drupal\language\Form;
+
+use Drupal\Core\Form\ConfigFormBaseTrait;
+use Drupal\Core\Form\ConfirmFormBase;
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Url;
+
+/**
+ * Defines a confirmation form for deleting a browser language negotiation mapping.
+ *
+ * @internal
+ */
+class NegotiationBrowserDeleteForm extends ConfirmFormBase {
+  use ConfigFormBaseTrait;
+
+  /**
+   * The browser language code to be deleted.
+   *
+   * @var string
+   */
+  protected $browserLangcode;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function getEditableConfigNames() {
+    return ['language.mappings'];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getQuestion() {
+    return $this->t('Are you sure you want to delete %browser_langcode?', ['%browser_langcode' => $this->browserLangcode]);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getCancelUrl() {
+    return new Url('language.negotiation_browser');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFormId() {
+    return 'language_negotiation_configure_browser_delete_form';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildForm(array $form, FormStateInterface $form_state, $browser_langcode = NULL) {
+    $this->browserLangcode = $browser_langcode;
+
+    $form = parent::buildForm($form, $form_state);
+
+    return $form;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function submitForm(array &$form, FormStateInterface $form_state) {
+    $this->config('language.mappings')
+      ->clear('map.' . $this->browserLangcode)
+      ->save();
+
+    $args = [
+      '%browser' => $this->browserLangcode,
+    ];
+
+    $this->logger('language')->notice('The browser language detection mapping for the %browser browser language code has been deleted.', $args);
+
+    $this->messenger()->addStatus($this->t('The mapping for the %browser browser language code has been deleted.', $args));
+
+    $form_state->setRedirect('language.negotiation_browser');
+  }
+
+}

web/core/modules/language/src/Form/NegotiationBrowserForm.php

@@ -0,0 +1,216 @@
+<?php
+
+namespace Drupal\language\Form;
+
+use Drupal\Core\Config\ConfigFactoryInterface;
+use Drupal\Core\Config\TypedConfigManagerInterface;
+use Drupal\Core\Form\ConfigFormBase;
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Url;
+use Drupal\language\ConfigurableLanguageManagerInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Configure the browser language negotiation method for this site.
+ *
+ * @internal
+ */
+class NegotiationBrowserForm extends ConfigFormBase {
+
+  /**
+   * The configurable language manager.
+   *
+   * @var \Drupal\language\ConfigurableLanguageManagerInterface
+   */
+  protected $languageManager;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function __construct(ConfigFactoryInterface $config_factory, TypedConfigManagerInterface $typedConfigManager, ConfigurableLanguageManagerInterface $language_manager) {
+    parent::__construct($config_factory, $typedConfigManager);
+    $this->languageManager = $language_manager;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container) {
+    return new static(
+      $container->get('config.factory'),
+      $container->get('config.typed'),
+      $container->get('language_manager')
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFormId() {
+    return 'language_negotiation_configure_browser_form';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function getEditableConfigNames() {
+    return ['language.mappings'];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildForm(array $form, FormStateInterface $form_state) {
+    $form = [];
+
+    // Initialize a language list to the ones available, including English.
+    $languages = $this->languageManager->getLanguages();
+
+    $existing_languages = [];
+    foreach ($languages as $langcode => $language) {
+      $existing_languages[$langcode] = $language->getName();
+    }
+
+    // If we have no languages available, present the list of predefined languages
+    // only. If we do have already added languages, set up two option groups with
+    // the list of existing and then predefined languages.
+    if (empty($existing_languages)) {
+      $language_options = $this->languageManager->getStandardLanguageListWithoutConfigured();
+    }
+    else {
+      $language_options = [
+        (string) $this->t('Existing languages') => $existing_languages,
+        (string) $this->t('Languages not yet added') => $this->languageManager->getStandardLanguageListWithoutConfigured(),
+      ];
+    }
+
+    $form['mappings'] = [
+      '#type' => 'table',
+      '#header' => [
+        $this->t('Browser language code'),
+        $this->t('Site language'),
+        $this->t('Operations'),
+      ],
+      '#attributes' => ['id' => 'language-negotiation-browser'],
+      '#empty' => $this->t('No browser language mappings available.'),
+    ];
+
+    $mappings = $this->language_get_browser_drupal_langcode_mappings();
+    foreach ($mappings as $browser_langcode => $drupal_langcode) {
+      $form['mappings'][$browser_langcode] = [
+        'browser_langcode' => [
+          '#title' => $this->t('Browser language code'),
+          '#title_display' => 'invisible',
+          '#type' => 'textfield',
+          '#default_value' => $browser_langcode,
+          '#size' => 20,
+          '#required' => TRUE,
+        ],
+        'drupal_langcode' => [
+          '#title' => $this->t('Site language'),
+          '#title_display' => 'invisible',
+          '#type' => 'select',
+          '#options' => $language_options,
+          '#default_value' => $drupal_langcode,
+          '#required' => TRUE,
+        ],
+      ];
+      // Operations column.
+      $form['mappings'][$browser_langcode]['operations'] = [
+        '#type' => 'operations',
+        '#links' => [],
+      ];
+      $form['mappings'][$browser_langcode]['operations']['#links']['delete'] = [
+        'title' => $this->t('Delete'),
+        'url' => Url::fromRoute('language.negotiation_browser_delete', ['browser_langcode' => $browser_langcode]),
+      ];
+    }
+
+    // Add empty row.
+    $form['new_mapping'] = [
+      '#type' => 'details',
+      '#title' => $this->t('Add a new mapping'),
+      '#tree' => TRUE,
+    ];
+    $form['new_mapping']['browser_langcode'] = [
+      '#type' => 'textfield',
+      '#title' => $this->t('Browser language code'),
+      '#description' => $this->t('Use language codes as <a href=":w3ctags">defined by the W3C</a> for interoperability. <em>Examples: "en", "en-gb" and "zh-hant".</em>', [':w3ctags' => 'http://www.w3.org/International/articles/language-tags/']),
+      '#size' => 20,
+    ];
+    $form['new_mapping']['drupal_langcode'] = [
+      '#type' => 'select',
+      '#title' => $this->t('Site language'),
+      '#options' => $language_options,
+    ];
+
+    return parent::buildForm($form, $form_state);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function validateForm(array &$form, FormStateInterface $form_state) {
+    // Array to check if all browser language codes are unique.
+    $unique_values = [];
+
+    // Check all mappings.
+    if ($form_state->hasValue('mappings')) {
+      $mappings = $form_state->getValue('mappings');
+      foreach ($mappings as $key => $data) {
+        // Make sure browser_langcode is unique.
+        if (array_key_exists($data['browser_langcode'], $unique_values)) {
+          $form_state->setErrorByName('mappings][new_mapping][browser_langcode', $this->t('Browser language codes must be unique.'));
+        }
+        elseif (preg_match('/[^a-z\-]/', $data['browser_langcode'])) {
+          $form_state->setErrorByName('mappings][new_mapping][browser_langcode', $this->t('Browser language codes can only contain lowercase letters and a hyphen(-).'));
+        }
+        $unique_values[$data['browser_langcode']] = $data['drupal_langcode'];
+      }
+    }
+
+    // Check new mapping.
+    $data = $form_state->getValue('new_mapping');
+    if (!empty($data['browser_langcode'])) {
+      // Make sure browser_langcode is unique.
+      if (array_key_exists($data['browser_langcode'], $unique_values)) {
+        $form_state->setErrorByName('mappings][' . $key . '][browser_langcode', $this->t('Browser language codes must be unique.'));
+      }
+      elseif (preg_match('/[^a-z\-]/', $data['browser_langcode'])) {
+        $form_state->setErrorByName('mappings][' . $key . '][browser_langcode', $this->t('Browser language codes can only contain lowercase letters and a hyphen(-).'));
+      }
+      $unique_values[$data['browser_langcode']] = $data['drupal_langcode'];
+    }
+
+    $form_state->set('mappings', $unique_values);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function submitForm(array &$form, FormStateInterface $form_state) {
+    $mappings = $form_state->get('mappings');
+    if (!empty($mappings)) {
+      $config = $this->config('language.mappings');
+      $config->setData(['map' => $mappings]);
+      $config->save();
+    }
+
+    parent::submitForm($form, $form_state);
+  }
+
+  /**
+   * Retrieves the browser's langcode mapping configuration array.
+   *
+   * @return array
+   *   The browser's langcode mapping configuration array.
+   */
+  protected function language_get_browser_drupal_langcode_mappings() {
+    $config = $this->config('language.mappings');
+    if ($config->isNew()) {
+      return [];
+    }
+    return $config->get('map');
+  }
+
+}

web/core/modules/language/src/Form/NegotiationConfigureForm.php

@@ -0,0 +1,342 @@
+<?php
+
+namespace Drupal\language\Form;
+
+use Drupal\Core\Block\BlockManagerInterface;
+use Drupal\Core\Config\ConfigFactoryInterface;
+use Drupal\Core\Config\TypedConfigManagerInterface;
+use Drupal\Core\Entity\EntityStorageInterface;
+use Drupal\Core\Extension\ThemeHandlerInterface;
+use Drupal\Core\Form\ConfigFormBase;
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Url;
+use Drupal\language\ConfigurableLanguageManagerInterface;
+use Drupal\language\LanguageNegotiatorInterface;
+use Drupal\language\Plugin\LanguageNegotiation\LanguageNegotiationSelected;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Configure the selected language negotiation method for this site.
+ *
+ * @internal
+ */
+class NegotiationConfigureForm extends ConfigFormBase {
+
+  /**
+   * Stores the configuration object for language.types.
+   *
+   * @var \Drupal\Core\Config\Config
+   */
+  protected $languageTypes;
+
+  /**
+   * The language manager.
+   *
+   * @var \Drupal\language\ConfigurableLanguageManagerInterface
+   */
+  protected $languageManager;
+
+  /**
+   * The language negotiator.
+   *
+   * @var \Drupal\language\LanguageNegotiatorInterface
+   */
+  protected $negotiator;
+
+  /**
+   * The block manager.
+   *
+   * @var \Drupal\Core\Block\BlockManagerInterface
+   */
+  protected $blockManager;
+
+  /**
+   * The block storage.
+   *
+   * @var \Drupal\Core\Entity\EntityStorageInterface|null
+   */
+  protected $blockStorage;
+
+  /**
+   * The theme handler.
+   *
+   * @var \Drupal\Core\Extension\ThemeHandlerInterface
+   */
+  protected $themeHandler;
+
+  /**
+   * Constructs a NegotiationConfigureForm object.
+   *
+   * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
+   *   The factory for configuration objects.
+   * @param \Drupal\Core\Config\TypedConfigManagerInterface $typedConfigManager
+   *   The typed config manager.
+   * @param \Drupal\language\ConfigurableLanguageManagerInterface $language_manager
+   *   The language manager.
+   * @param \Drupal\language\LanguageNegotiatorInterface $negotiator
+   *   The language negotiation methods manager.
+   * @param \Drupal\Core\Block\BlockManagerInterface $block_manager
+   *   The block manager.
+   * @param \Drupal\Core\Extension\ThemeHandlerInterface $theme_handler
+   *   The theme handler.
+   * @param \Drupal\Core\Entity\EntityStorageInterface $block_storage
+   *   The block storage, or NULL if not available.
+   */
+  public function __construct(ConfigFactoryInterface $config_factory, TypedConfigManagerInterface $typedConfigManager, ConfigurableLanguageManagerInterface $language_manager, LanguageNegotiatorInterface $negotiator, BlockManagerInterface $block_manager, ThemeHandlerInterface $theme_handler, ?EntityStorageInterface $block_storage = NULL) {
+    parent::__construct($config_factory, $typedConfigManager);
+    $this->languageTypes = $this->config('language.types');
+    $this->languageManager = $language_manager;
+    $this->negotiator = $negotiator;
+    $this->blockManager = $block_manager;
+    $this->themeHandler = $theme_handler;
+    $this->blockStorage = $block_storage;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container) {
+    $entity_type_manager = $container->get('entity_type.manager');
+    $block_storage = $entity_type_manager->hasHandler('block', 'storage') ? $entity_type_manager->getStorage('block') : NULL;
+    return new static(
+      $container->get('config.factory'),
+      $container->get('config.typed'),
+      $container->get('language_manager'),
+      $container->get('language_negotiator'),
+      $container->get('plugin.manager.block'),
+      $container->get('theme_handler'),
+      $block_storage
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFormId() {
+    return 'language_negotiation_configure_form';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function getEditableConfigNames() {
+    return ['language.types'];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildForm(array $form, FormStateInterface $form_state) {
+    $configurable = $this->languageTypes->get('configurable');
+
+    $form = [
+      '#theme' => 'language_negotiation_configure_form',
+      '#language_types_info' => $this->languageManager->getDefinedLanguageTypesInfo(),
+      '#language_negotiation_info' => $this->negotiator->getNegotiationMethods(),
+    ];
+    $form['#language_types'] = [];
+
+    foreach ($form['#language_types_info'] as $type => $info) {
+      // Show locked language types only if they are configurable.
+      if (empty($info['locked']) || in_array($type, $configurable)) {
+        $form['#language_types'][] = $type;
+      }
+    }
+
+    foreach ($form['#language_types'] as $type) {
+      $this->configureFormTable($form, $type);
+    }
+
+    $form['actions'] = ['#type' => 'actions'];
+    $form['actions']['submit'] = [
+      '#type' => 'submit',
+      '#button_type' => 'primary',
+      '#value' => $this->t('Save settings'),
+    ];
+
+    return $form;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function submitForm(array &$form, FormStateInterface $form_state) {
+    $configurable_types = $form['#language_types'];
+
+    $stored_values = $this->languageTypes->get('configurable');
+    $customized = [];
+    $method_weights_type = [];
+
+    foreach ($configurable_types as $type) {
+      $customized[$type] = in_array($type, $stored_values);
+      $method_weights = [];
+      $enabled_methods = $form_state->getValue([$type, 'enabled']);
+      $enabled_methods[LanguageNegotiationSelected::METHOD_ID] = TRUE;
+      $method_weights_input = $form_state->getValue([$type, 'weight']);
+      if ($form_state->hasValue([$type, 'configurable'])) {
+        $customized[$type] = !$form_state->isValueEmpty([$type, 'configurable']);
+      }
+
+      foreach ($method_weights_input as $method_id => $weight) {
+        if ($enabled_methods[$method_id]) {
+          $method_weights[$method_id] = $weight;
+        }
+      }
+
+      $method_weights_type[$type] = $method_weights;
+      $this->languageTypes->set('negotiation.' . $type . '.method_weights', $method_weights_input)->save();
+    }
+
+    // Update non-configurable language types and the related language
+    // negotiation configuration.
+    $this->negotiator->updateConfiguration(array_keys(array_filter($customized)));
+
+    // Update the language negotiations after setting the configurability.
+    foreach ($method_weights_type as $type => $method_weights) {
+      $this->negotiator->saveConfiguration($type, $method_weights);
+    }
+
+    // Clear block definitions cache since the available blocks and their names
+    // may have been changed based on the configurable types.
+    if ($this->blockStorage) {
+      // If there is an active language switcher for a language type that has
+      // been made not configurable, deactivate it first.
+      $non_configurable = array_keys(array_diff($customized, array_filter($customized)));
+      $this->disableLanguageSwitcher($non_configurable);
+    }
+    $this->blockManager->clearCachedDefinitions();
+
+    $form_state->setRedirect('language.negotiation');
+    $this->messenger()->addStatus($this->t('Language detection configuration saved.'));
+  }
+
+  /**
+   * Builds a language negotiation method configuration table.
+   *
+   * @param array $form
+   *   The language negotiation configuration form.
+   * @param string $type
+   *   The language type to generate the table for.
+   */
+  protected function configureFormTable(array &$form, $type) {
+    $info = $form['#language_types_info'][$type];
+
+    $table_form = [
+      '#title' => $this->t('@type language detection', ['@type' => $info['name']]),
+      '#tree' => TRUE,
+      '#description' => $info['description'],
+      '#language_negotiation_info' => [],
+      '#show_operations' => FALSE,
+      'weight' => ['#tree' => TRUE],
+    ];
+    // Only show configurability checkbox for the unlocked language types.
+    if (empty($info['locked'])) {
+      $configurable = $this->languageTypes->get('configurable');
+      $table_form['configurable'] = [
+        '#type' => 'checkbox',
+        '#title' => $this->t('Customize %language_name language detection to differ from Interface text language detection settings', ['%language_name' => $info['name']]),
+        '#default_value' => in_array($type, $configurable),
+        '#attributes' => ['class' => ['language-customization-checkbox']],
+        '#attached' => [
+          'library' => [
+            'language/drupal.language.admin',
+          ],
+        ],
+      ];
+    }
+
+    $negotiation_info = $form['#language_negotiation_info'];
+    $enabled_methods = $this->languageTypes->get('negotiation.' . $type . '.enabled') ?: [];
+    $methods_weight = $this->languageTypes->get('negotiation.' . $type . '.method_weights') ?: [];
+
+    // Add missing data to the methods lists.
+    foreach ($negotiation_info as $method_id => $method) {
+      if (!isset($methods_weight[$method_id])) {
+        $methods_weight[$method_id] = $method['weight'] ?? 0;
+      }
+    }
+
+    // Order methods list by weight.
+    asort($methods_weight);
+
+    foreach ($methods_weight as $method_id => $weight) {
+      // A language method might be no more available if the defining module has
+      // been disabled after the last configuration saving.
+      if (!isset($negotiation_info[$method_id])) {
+        continue;
+      }
+
+      $enabled = isset($enabled_methods[$method_id]);
+      $method = $negotiation_info[$method_id];
+
+      // List the method only if the current type is defined in its 'types' key.
+      // If it is not defined default to all the configurable language types.
+      $types = array_flip($method['types'] ?? $form['#language_types']);
+
+      if (isset($types[$type])) {
+        $table_form['#language_negotiation_info'][$method_id] = $method;
+        $method_name = $method['name'];
+
+        $table_form['weight'][$method_id] = [
+          '#type' => 'weight',
+          '#title' => $this->t('Weight for @title language detection method', ['@title' => mb_strtolower($method_name)]),
+          '#title_display' => 'invisible',
+          '#default_value' => $weight,
+          '#attributes' => ['class' => ["language-method-weight-$type"]],
+          '#delta' => 20,
+        ];
+
+        $table_form['title'][$method_id] = ['#plain_text' => $method_name];
+
+        $table_form['enabled'][$method_id] = [
+          '#type' => 'checkbox',
+          '#title' => $this->t('Enable @title language detection method', ['@title' => mb_strtolower($method_name)]),
+          '#title_display' => 'invisible',
+          '#default_value' => $enabled,
+        ];
+        if ($method_id === LanguageNegotiationSelected::METHOD_ID) {
+          $table_form['enabled'][$method_id]['#default_value'] = TRUE;
+          $table_form['enabled'][$method_id]['#attributes'] = ['disabled' => 'disabled'];
+        }
+
+        $table_form['description'][$method_id] = ['#markup' => $method['description']];
+
+        $config_op = [];
+        if (isset($method['config_route_name'])) {
+          $config_op['configure'] = [
+            'title' => $this->t('Configure'),
+            'url' => Url::fromRoute($method['config_route_name']),
+          ];
+          // If there is at least one operation enabled show the operation
+          // column.
+          $table_form['#show_operations'] = TRUE;
+        }
+        $table_form['operation'][$method_id] = [
+          '#type' => 'operations',
+          '#links' => $config_op,
+        ];
+      }
+    }
+    $form[$type] = $table_form;
+  }
+
+  /**
+   * Disables the language switcher blocks.
+   *
+   * @param array $language_types
+   *   An array containing all language types whose language switchers need to
+   *   be disabled.
+   */
+  protected function disableLanguageSwitcher(array $language_types) {
+    $theme = $this->themeHandler->getDefault();
+    $blocks = $this->blockStorage->loadByProperties(['theme' => $theme]);
+    foreach ($language_types as $language_type) {
+      foreach ($blocks as $block) {
+        if ($block->getPluginId() == 'language_block:' . $language_type) {
+          $block->delete();
+        }
+      }
+    }
+  }
+
+}

web/core/modules/language/src/Form/NegotiationSelectedForm.php

@@ -0,0 +1,56 @@
+<?php
+
+namespace Drupal\language\Form;
+
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Language\LanguageInterface;
+use Drupal\Core\Form\ConfigFormBase;
+
+/**
+ * Configure the selected language negotiation method for this site.
+ *
+ * @internal
+ */
+class NegotiationSelectedForm extends ConfigFormBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFormId() {
+    return 'language_negotiation_configure_selected_form';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function getEditableConfigNames() {
+    return ['language.negotiation'];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildForm(array $form, FormStateInterface $form_state) {
+    $config = $this->config('language.negotiation');
+    $form['selected_langcode'] = [
+      '#type' => 'language_select',
+      '#title' => $this->t('Language'),
+      '#languages' => LanguageInterface::STATE_CONFIGURABLE | LanguageInterface::STATE_SITE_DEFAULT,
+      '#default_value' => $config->get('selected_langcode'),
+    ];
+
+    return parent::buildForm($form, $form_state);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function submitForm(array &$form, FormStateInterface $form_state) {
+    $this->config('language.negotiation')
+      ->set('selected_langcode', $form_state->getValue('selected_langcode'))
+      ->save();
+
+    parent::submitForm($form, $form_state);
+  }
+
+}

web/core/modules/language/src/Form/NegotiationSessionForm.php

@@ -0,0 +1,57 @@
+<?php
+
+namespace Drupal\language\Form;
+
+use Drupal\Core\Form\ConfigFormBase;
+use Drupal\Core\Form\FormStateInterface;
+
+/**
+ * Configure the session language negotiation method for this site.
+ *
+ * @internal
+ */
+class NegotiationSessionForm extends ConfigFormBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFormId() {
+    return 'language_negotiation_configure_session_form';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function getEditableConfigNames() {
+    return ['language.negotiation'];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildForm(array $form, FormStateInterface $form_state) {
+    $config = $this->config('language.negotiation');
+    $form['language_negotiation_session_param'] = [
+      '#title' => $this->t('Request/session parameter'),
+      '#type' => 'textfield',
+      '#default_value' => $config->get('session.parameter'),
+      '#description' => $this->t('Name of the request/session parameter used to determine the desired language.'),
+    ];
+
+    $form_state->setRedirect('language.negotiation');
+
+    return parent::buildForm($form, $form_state);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function submitForm(array &$form, FormStateInterface $form_state) {
+    $this->config('language.negotiation')
+      ->set('session.parameter', $form_state->getValue('language_negotiation_session_param'))
+      ->save();
+
+    parent::submitForm($form, $form_state);
+  }
+
+}

web/core/modules/language/src/Form/NegotiationUrlForm.php

@@ -0,0 +1,226 @@
+<?php
+
+namespace Drupal\language\Form;
+
+use Drupal\Core\Config\TypedConfigManagerInterface;
+use Drupal\Core\Form\ConfigFormBase;
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Language\LanguageInterface;
+use Drupal\Core\Language\LanguageManagerInterface;
+use Drupal\Core\Config\ConfigFactoryInterface;
+use Drupal\Core\Url;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+use Drupal\language\Plugin\LanguageNegotiation\LanguageNegotiationUrl;
+
+// cspell:ignore deutsch
+
+/**
+ * Configure the URL language negotiation method for this site.
+ *
+ * @internal
+ */
+class NegotiationUrlForm extends ConfigFormBase {
+
+  /**
+   * The language manager.
+   *
+   * @var \Drupal\Core\Language\LanguageManagerInterface
+   */
+  protected $languageManager;
+
+  /**
+   * Constructs a new NegotiationUrlForm object.
+   *
+   * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
+   *   The factory for configuration objects.
+   * @param \Drupal\Core\Config\TypedConfigManagerInterface $typedConfigManager
+   *   The typed config manager.
+   * @param \Drupal\Core\Language\LanguageManagerInterface $language_manager
+   *   The language manager.
+   */
+  public function __construct(ConfigFactoryInterface $config_factory, TypedConfigManagerInterface $typedConfigManager, LanguageManagerInterface $language_manager) {
+    parent::__construct($config_factory, $typedConfigManager);
+    $this->languageManager = $language_manager;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container) {
+    return new static(
+      $container->get('config.factory'),
+      $container->get('config.typed'),
+      $container->get('language_manager')
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFormId() {
+    return 'language_negotiation_configure_url_form';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function getEditableConfigNames() {
+    return ['language.negotiation'];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildForm(array $form, FormStateInterface $form_state) {
+    global $base_url;
+    $config = $this->config('language.negotiation');
+
+    $form['language_negotiation_url_part'] = [
+      '#title' => $this->t('Part of the URL that determines language'),
+      '#type' => 'radios',
+      '#options' => [
+        LanguageNegotiationUrl::CONFIG_PATH_PREFIX => $this->t('Path prefix'),
+        LanguageNegotiationUrl::CONFIG_DOMAIN => $this->t('Domain'),
+      ],
+      '#default_value' => $config->get('url.source'),
+    ];
+
+    $form['prefix'] = [
+      '#type' => 'details',
+      '#tree' => TRUE,
+      '#title' => $this->t('Path prefix configuration'),
+      '#open' => TRUE,
+      '#description' => $this->t('Language codes or other custom text to use as a path prefix for URL language detection. For the selected fallback language, this value may be left blank. <strong>Modifying this value may break existing URLs. Use with caution in a production environment.</strong> Example: Specifying "deutsch" as the path prefix code for German results in URLs like "example.com/deutsch/contact".'),
+      '#states' => [
+        'visible' => [
+          ':input[name="language_negotiation_url_part"]' => [
+            'value' => (string) LanguageNegotiationUrl::CONFIG_PATH_PREFIX,
+          ],
+        ],
+      ],
+    ];
+    $form['domain'] = [
+      '#type' => 'details',
+      '#tree' => TRUE,
+      '#title' => $this->t('Domain configuration'),
+      '#open' => TRUE,
+      '#description' => $this->t('The domain names to use for these languages. <strong>Modifying this value may break existing URLs. Use with caution in a production environment.</strong> Example: Specifying "de.example.com" as language domain for German will result in a URL like "http://de.example.com/contact".'),
+      '#states' => [
+        'visible' => [
+          ':input[name="language_negotiation_url_part"]' => [
+            'value' => (string) LanguageNegotiationUrl::CONFIG_DOMAIN,
+          ],
+        ],
+      ],
+    ];
+
+    $languages = $this->languageManager->getLanguages();
+    $prefixes = $config->get('url.prefixes');
+    $domains = $config->get('url.domains');
+    foreach ($languages as $langcode => $language) {
+      $t_args = ['%language' => $language->getName(), '%langcode' => $language->getId()];
+      $form['prefix'][$langcode] = [
+        '#type' => 'textfield',
+        '#title' => $language->isDefault() ? $this->t('%language (%langcode) path prefix (Default language)', $t_args) : $this->t('%language (%langcode) path prefix', $t_args),
+        '#maxlength' => 64,
+        '#default_value' => $prefixes[$langcode] ?? '',
+        '#field_prefix' => $base_url . '/',
+      ];
+      $form['domain'][$langcode] = [
+        '#type' => 'textfield',
+        '#title' => $this->t('%language (%langcode) domain', ['%language' => $language->getName(), '%langcode' => $language->getId()]),
+        '#maxlength' => 128,
+        '#default_value' => $domains[$langcode] ?? '',
+      ];
+    }
+
+    $form_state->setRedirect('language.negotiation');
+
+    return parent::buildForm($form, $form_state);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function validateForm(array &$form, FormStateInterface $form_state) {
+    $languages = $this->languageManager->getLanguages();
+
+    // Count repeated values for uniqueness check.
+    $count = array_count_values($form_state->getValue('prefix'));
+    $default_langcode = $this->config('language.negotiation')->get('selected_langcode');
+    if ($default_langcode == LanguageInterface::LANGCODE_SITE_DEFAULT) {
+      $default_langcode = $this->languageManager->getDefaultLanguage()->getId();
+    }
+    foreach ($languages as $langcode => $language) {
+      $value = $form_state->getValue(['prefix', $langcode]);
+      if ($value === '') {
+        if (!($default_langcode == $langcode) && $form_state->getValue('language_negotiation_url_part') == LanguageNegotiationUrl::CONFIG_PATH_PREFIX) {
+          // Throw a form error if the prefix is blank for a non-default language,
+          // although it is required for selected negotiation type.
+          $form_state->setErrorByName("prefix][$langcode", $this->t('The prefix may only be left blank for the <a href=":url">selected detection fallback language.</a>', [
+            ':url' => Url::fromRoute('language.negotiation_selected')->toString(),
+          ]));
+        }
+      }
+      elseif (str_contains($value, '/')) {
+        // Throw a form error if the string contains a slash,
+        // which would not work.
+        $form_state->setErrorByName("prefix][$langcode", $this->t('The prefix may not contain a slash.'));
+      }
+      elseif (isset($count[$value]) && $count[$value] > 1) {
+        // Throw a form error if there are two languages with the same
+        // domain/prefix.
+        $form_state->setErrorByName("prefix][$langcode", $this->t('The prefix for %language, %value, is not unique.', ['%language' => $language->getName(), '%value' => $value]));
+      }
+    }
+
+    // Count repeated values for uniqueness check.
+    $count = array_count_values($form_state->getValue('domain'));
+    foreach ($languages as $langcode => $language) {
+      $value = $form_state->getValue(['domain', $langcode]);
+
+      if ($value === '') {
+        if ($form_state->getValue('language_negotiation_url_part') == LanguageNegotiationUrl::CONFIG_DOMAIN) {
+          // Throw a form error if the domain is blank for a non-default language,
+          // although it is required for selected negotiation type.
+          $form_state->setErrorByName("domain][$langcode", $this->t('The domain may not be left blank for %language.', ['%language' => $language->getName()]));
+        }
+      }
+      elseif (isset($count[$value]) && $count[$value] > 1) {
+        // Throw a form error if there are two languages with the same
+        // domain/domain.
+        $form_state->setErrorByName("domain][$langcode", $this->t('The domain for %language, %value, is not unique.', ['%language' => $language->getName(), '%value' => $value]));
+      }
+    }
+
+    // Domain names should not contain protocol and/or ports.
+    foreach ($languages as $langcode => $language) {
+      $value = $form_state->getValue(['domain', $langcode]);
+      if (!empty($value)) {
+        // Ensure we have exactly one protocol when checking the hostname.
+        $host = 'http://' . str_replace(['http://', 'https://'], '', $value);
+        if (parse_url($host, PHP_URL_HOST) != $value) {
+          $form_state->setErrorByName("domain][$langcode", $this->t('The domain for %language may only contain the domain name, not a trailing slash, protocol and/or port.', ['%language' => $language->getName()]));
+        }
+      }
+    }
+
+    parent::validateForm($form, $form_state);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function submitForm(array &$form, FormStateInterface $form_state) {
+    // Save selected format (prefix or domain).
+    $this->config('language.negotiation')
+      ->set('url.source', $form_state->getValue('language_negotiation_url_part'))
+      // Save new domain and prefix values.
+      ->set('url.prefixes', $form_state->getValue('prefix'))
+      ->set('url.domains', $form_state->getValue('domain'))
+      ->save();
+
+    parent::submitForm($form, $form_state);
+  }
+
+}

web/core/modules/language/src/HttpKernel/PathProcessorLanguage.php

@@ -0,0 +1,171 @@
+<?php
+
+namespace Drupal\language\HttpKernel;
+
+use Drupal\Component\Utility\Unicode;
+use Drupal\Core\Config\ConfigFactoryInterface;
+use Drupal\Core\PathProcessor\InboundPathProcessorInterface;
+use Drupal\Core\PathProcessor\OutboundPathProcessorInterface;
+use Drupal\Core\Render\BubbleableMetadata;
+use Drupal\language\ConfigurableLanguageManagerInterface;
+use Drupal\language\EventSubscriber\ConfigSubscriber;
+use Drupal\language\LanguageNegotiatorInterface;
+use Symfony\Component\HttpFoundation\Request;
+use Drupal\Core\Session\AccountInterface;
+
+/**
+ * Processes the inbound path using path alias lookups.
+ */
+class PathProcessorLanguage implements InboundPathProcessorInterface, OutboundPathProcessorInterface {
+
+  /**
+   * A config factory for retrieving required config settings.
+   *
+   * @var \Drupal\Core\Config\ConfigFactoryInterface
+   */
+  protected $config;
+
+  /**
+   * Language manager for retrieving the URL language type.
+   *
+   * @var \Drupal\language\ConfigurableLanguageManagerInterface
+   */
+  protected $languageManager;
+
+  /**
+   * The language negotiator.
+   *
+   * @var \Drupal\language\LanguageNegotiatorInterface
+   */
+  protected $negotiator;
+
+  /**
+   * Local cache for language path processors.
+   *
+   * @var array
+   */
+  protected $processors;
+
+  /**
+   * Flag indicating whether the site is multilingual.
+   *
+   * @var bool
+   */
+  protected $multilingual;
+
+  /**
+   * The language configuration event subscriber.
+   *
+   * @var \Drupal\language\EventSubscriber\ConfigSubscriber
+   */
+  protected $configSubscriber;
+
+  /**
+   * Constructs a PathProcessorLanguage object.
+   *
+   * @param \Drupal\Core\Config\ConfigFactoryInterface $config
+   *   A config factory object for retrieving configuration settings.
+   * @param \Drupal\language\ConfigurableLanguageManagerInterface $language_manager
+   *   The configurable language manager.
+   * @param \Drupal\language\LanguageNegotiatorInterface $negotiator
+   *   The language negotiator.
+   * @param \Drupal\Core\Session\AccountInterface $current_user
+   *   The current active user.
+   * @param \Drupal\language\EventSubscriber\ConfigSubscriber $config_subscriber
+   *   The language configuration event subscriber.
+   */
+  public function __construct(ConfigFactoryInterface $config, ConfigurableLanguageManagerInterface $language_manager, LanguageNegotiatorInterface $negotiator, AccountInterface $current_user, ConfigSubscriber $config_subscriber) {
+    $this->config = $config;
+    $this->languageManager = $language_manager;
+    $this->negotiator = $negotiator;
+    $this->negotiator->setCurrentUser($current_user);
+    $this->configSubscriber = $config_subscriber;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function processInbound($path, Request $request) {
+    if (!empty($path)) {
+      $scope = 'inbound';
+      if (!isset($this->processors[$scope])) {
+        $this->initProcessors($scope);
+      }
+      foreach ($this->processors[$scope] as $instance) {
+        $path = $instance->processInbound($path, $request);
+      }
+    }
+    return $path;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function processOutbound($path, &$options = [], ?Request $request = NULL, ?BubbleableMetadata $bubbleable_metadata = NULL) {
+    if (!isset($this->multilingual)) {
+      $this->multilingual = $this->languageManager->isMultilingual();
+    }
+    if ($this->multilingual) {
+      $this->negotiator->reset();
+      $scope = 'outbound';
+      if (!isset($this->processors[$scope])) {
+        $this->initProcessors($scope);
+      }
+      foreach ($this->processors[$scope] as $instance) {
+        $path = $instance->processOutbound($path, $options, $request, $bubbleable_metadata);
+      }
+      // No language dependent path allowed in this mode.
+      if (empty($this->processors[$scope])) {
+        unset($options['language']);
+      }
+    }
+    return $path;
+  }
+
+  /**
+   * Initializes the local cache for language path processors.
+   *
+   * @param string $scope
+   *   The scope of the processors: "inbound" or "outbound".
+   */
+  protected function initProcessors($scope) {
+    $interface = 'Drupal\Core\PathProcessor\\' . Unicode::ucfirst($scope) . 'PathProcessorInterface';
+    $this->processors[$scope] = [];
+    $weights = [];
+    foreach ($this->languageManager->getLanguageTypes() as $type) {
+      foreach ($this->negotiator->getNegotiationMethods($type) as $method_id => $method) {
+        if (!isset($this->processors[$scope][$method_id])) {
+          if (is_subclass_of($method['class'], $interface)) {
+            $this->processors[$scope][$method_id] = $this->negotiator->getNegotiationMethodInstance($method_id);
+            $weights[$method_id] = $method['weight'];
+          }
+        }
+      }
+    }
+
+    // Sort the processors list, so that their functions are called in the
+    // order specified by the weight of the methods.
+    uksort($this->processors[$scope], function ($method_id_a, $method_id_b) use ($weights) {
+      return $weights[$method_id_a] <=> $weights[$method_id_b];
+    });
+  }
+
+  /**
+   * Initializes the injected event subscriber with the language path processor.
+   *
+   * The language path processor service is registered only on multilingual
+   * site configuration, thus we inject it in the event subscriber only when
+   * it is initialized.
+   */
+  public function initConfigSubscriber() {
+    $this->configSubscriber->setPathProcessorLanguage($this);
+  }
+
+  /**
+   * Resets the collected processors instances.
+   */
+  public function reset() {
+    $this->processors = [];
+  }
+
+}

web/core/modules/language/src/LanguageAccessControlHandler.php

@@ -0,0 +1,42 @@
+<?php
+
+namespace Drupal\language;
+
+use Drupal\Core\Access\AccessResult;
+use Drupal\Core\Entity\EntityAccessControlHandler;
+use Drupal\Core\Entity\EntityInterface;
+use Drupal\Core\Session\AccountInterface;
+
+/**
+ * Defines the access control handler for the language entity type.
+ *
+ * @see \Drupal\language\Entity\ConfigurableLanguage
+ */
+class LanguageAccessControlHandler extends EntityAccessControlHandler {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function checkAccess(EntityInterface $entity, $operation, AccountInterface $account) {
+    switch ($operation) {
+      case 'view':
+        return parent::checkAccess($entity, $operation, $account);
+
+      case 'update':
+        /** @var \Drupal\Core\Language\LanguageInterface $entity */
+        return AccessResult::allowedIf(!$entity->isLocked())->addCacheableDependency($entity)
+          ->andIf(parent::checkAccess($entity, $operation, $account));
+
+      case 'delete':
+        /** @var \Drupal\Core\Language\LanguageInterface $entity */
+        return AccessResult::allowedIf(!$entity->isLocked())->addCacheableDependency($entity)
+          ->andIf(AccessResult::allowedIf(!$entity->isDefault())->addCacheableDependency($entity))
+          ->andIf(parent::checkAccess($entity, $operation, $account));
+
+      default:
+        // No opinion.
+        return AccessResult::neutral();
+    }
+  }
+
+}

web/core/modules/language/src/LanguageConverter.php

@@ -0,0 +1,48 @@
+<?php
+
+namespace Drupal\language;
+
+use Drupal\Core\Language\LanguageManagerInterface;
+use Drupal\Core\ParamConverter\ParamConverterInterface;
+use Symfony\Component\Routing\Route;
+
+/**
+ * Converts parameters for upcasting entity IDs to full objects.
+ */
+class LanguageConverter implements ParamConverterInterface {
+
+  /**
+   * The language manager.
+   *
+   * @var \Drupal\Core\Language\LanguageManagerInterface
+   */
+  protected $languageManager;
+
+  /**
+   * Constructs a new LanguageConverter.
+   *
+   * @param \Drupal\Core\Language\LanguageManagerInterface $language_manager
+   *   The language manager.
+   */
+  public function __construct(LanguageManagerInterface $language_manager) {
+    $this->languageManager = $language_manager;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function convert($value, $definition, $name, array $defaults) {
+    if (!empty($value)) {
+      return $this->languageManager->getLanguage($value);
+    }
+    return NULL;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function applies($definition, $name, Route $route) {
+    return (!empty($definition['type']) && $definition['type'] == 'language');
+  }
+
+}

web/core/modules/language/src/LanguageListBuilder.php

@@ -0,0 +1,175 @@
+<?php
+
+namespace Drupal\language;
+
+use Drupal\Core\Config\ConfigFactoryInterface;
+use Drupal\Core\Config\Entity\DraggableListBuilder;
+use Drupal\Core\Entity\EntityInterface;
+use Drupal\Core\Entity\EntityStorageInterface;
+use Drupal\Core\Entity\EntityTypeInterface;
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Language\LanguageManagerInterface;
+use Drupal\Core\Messenger\MessengerInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Defines a class to build a listing of language entities.
+ *
+ * @see \Drupal\language\Entity\ConfigurableLanguage
+ */
+class LanguageListBuilder extends DraggableListBuilder {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $entitiesKey = 'languages';
+
+  /**
+   * The language manager.
+   *
+   * @var \Drupal\Core\Language\LanguageManagerInterface
+   */
+  protected $languageManager;
+
+  /**
+   * The configuration factory.
+   *
+   * @var \Drupal\Core\Config\ConfigFactoryInterface
+   */
+  protected $configFactory;
+
+  /**
+   * The messenger.
+   *
+   * @var \Drupal\Core\Messenger\MessengerInterface
+   */
+  protected $messenger;
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function createInstance(ContainerInterface $container, EntityTypeInterface $entity_type) {
+    return new static(
+      $entity_type,
+      $container->get('entity_type.manager')->getStorage($entity_type->id()),
+      $container->get('language_manager'),
+      $container->get('config.factory'),
+      $container->get('messenger')
+    );
+  }
+
+  /**
+   * Constructs a new LanguageListBuilder object.
+   *
+   * @param \Drupal\Core\Entity\EntityTypeInterface $entity_type
+   *   The entity type definition.
+   * @param \Drupal\Core\Entity\EntityStorageInterface $storage
+   *   The entity storage handler class.
+   * @param \Drupal\Core\Language\LanguageManagerInterface $language_manager
+   *   The language manager.
+   * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
+   *   The factory for configuration objects.
+   * @param \Drupal\Core\Messenger\MessengerInterface $messenger
+   *   The messenger.
+   */
+  public function __construct(EntityTypeInterface $entity_type, EntityStorageInterface $storage, LanguageManagerInterface $language_manager, ConfigFactoryInterface $config_factory, MessengerInterface $messenger) {
+    parent::__construct($entity_type, $storage);
+    $this->languageManager = $language_manager;
+    $this->configFactory = $config_factory;
+    $this->messenger = $messenger;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function load() {
+    $entities = $this->storage->loadByProperties(['locked' => FALSE]);
+
+    // Sort the entities using the entity class's sort() method.
+    // See \Drupal\Core\Config\Entity\ConfigEntityBase::sort().
+    uasort($entities, [$this->entityType->getClass(), 'sort']);
+    return $entities;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFormId() {
+    return 'language_admin_overview_form';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildHeader() {
+    $header = [
+      'label' => t('Name'),
+      'default' => t('Default'),
+    ] + parent::buildHeader();
+    return $header;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildRow(EntityInterface $entity) {
+    $row['label'] = $entity->label();
+    $row['default'] = [
+      '#type' => 'radio',
+      '#parents' => ['site_default_language'],
+      '#title' => t('Set @title as default', ['@title' => $entity->label()]),
+      '#title_display' => 'invisible',
+      '#return_value' => $entity->id(),
+      '#id' => 'edit-site-default-language-' . $entity->id(),
+    ];
+    // Mark the right language as default in the form.
+    if ($entity->id() == $this->languageManager->getDefaultLanguage()->getId()) {
+      $row['default']['#default_value'] = $entity->id();
+    }
+    return $row + parent::buildRow($entity);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildForm(array $form, FormStateInterface $form_state) {
+    $form = parent::buildForm($form, $form_state);
+
+    $form[$this->entitiesKey]['#languages'] = $this->entities;
+    $form['actions']['submit']['#value'] = t('Save configuration');
+    return $form;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function validateForm(array &$form, FormStateInterface $form_state) {
+    if (!isset($this->entities[$form_state->getValue('site_default_language')])) {
+      $form_state->setErrorByName('site_default_language', $this->t('Selected default language no longer exists.'));
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function submitForm(array &$form, FormStateInterface $form_state) {
+    parent::submitForm($form, $form_state);
+
+    // Save the default language if changed.
+    $new_id = $form_state->getValue('site_default_language');
+    if ($new_id != $this->languageManager->getDefaultLanguage()->getId()) {
+      $this->configFactory->getEditable('system.site')->set('default_langcode', $new_id)->save();
+      $this->languageManager->reset();
+    }
+
+    if ($this->languageManager instanceof ConfigurableLanguageManagerInterface) {
+      $this->languageManager->updateLockedLanguageWeights();
+    }
+
+    $this->messenger->addStatus($this->t('Configuration saved.'));
+    // Force the redirection to the page with the language we have just
+    // selected as default.
+    $form_state->setRedirectUrl($this->entities[$new_id]->toUrl('collection', ['language' => $this->entities[$new_id]]));
+  }
+
+}

web/core/modules/language/src/LanguageNegotiationMethodBase.php

@@ -0,0 +1,63 @@
+<?php
+
+namespace Drupal\language;
+
+use Drupal\Core\Config\ConfigFactoryInterface;
+use Drupal\Core\Language\LanguageInterface;
+use Drupal\Core\Session\AccountInterface;
+
+/**
+ * Base class for language negotiation methods.
+ */
+abstract class LanguageNegotiationMethodBase implements LanguageNegotiationMethodInterface {
+
+  /**
+   * The language manager.
+   *
+   * @var \Drupal\Core\Language\LanguageManagerInterface
+   */
+  protected $languageManager;
+
+  /**
+   * The configuration factory.
+   *
+   * @var \Drupal\Core\Config\ConfigFactoryInterface
+   */
+  protected $config;
+
+  /**
+   * The current active user.
+   *
+   * @var \Drupal\Core\Session\AccountInterface
+   */
+  protected $currentUser;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setLanguageManager(ConfigurableLanguageManagerInterface $language_manager) {
+    $this->languageManager = $language_manager;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setConfig(ConfigFactoryInterface $config) {
+    $this->config = $config;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setCurrentUser(AccountInterface $current_user) {
+    $this->currentUser = $current_user;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function persist(LanguageInterface $language) {
+    // Default implementation persists nothing.
+  }
+
+}

web/core/modules/language/src/LanguageNegotiationMethodInterface.php

@@ -0,0 +1,67 @@
+<?php
+
+namespace Drupal\language;
+
+use Drupal\Core\Config\ConfigFactoryInterface;
+use Drupal\Core\Language\LanguageInterface;
+use Drupal\Core\Session\AccountInterface;
+use Symfony\Component\HttpFoundation\Request;
+
+/**
+ * Interface for language negotiation classes.
+ */
+interface LanguageNegotiationMethodInterface {
+
+  /**
+   * Injects the language manager.
+   *
+   * @param \Drupal\language\ConfigurableLanguageManagerInterface $language_manager
+   *   The language manager to be used to retrieve the language list and the
+   *   already negotiated languages.
+   */
+  public function setLanguageManager(ConfigurableLanguageManagerInterface $language_manager);
+
+  /**
+   * Injects the configuration factory.
+   *
+   * @param \Drupal\Core\Config\ConfigFactoryInterface $config
+   *   The configuration factory.
+   */
+  public function setConfig(ConfigFactoryInterface $config);
+
+  /**
+   * Injects the current user.
+   *
+   * @param \Drupal\Core\Session\AccountInterface $current_user
+   *   The current active user.
+   */
+  public function setCurrentUser(AccountInterface $current_user);
+
+  /**
+   * Performs language negotiation.
+   *
+   * @param \Symfony\Component\HttpFoundation\Request $request
+   *   (optional) The current request. Defaults to NULL if it has not been
+   *   initialized yet.
+   *
+   * @return string|null|false
+   *   A valid language code if the negotiation was successful and either NULL
+   *    or FALSE otherwise.
+   *
+   * @todo Determine whether string|false or string|null should be the
+   *   normalized result across all implementations and update the @return and
+   *   its comment accordingly.
+   *
+   * @see https://www.drupal.org/node/3329952
+   */
+  public function getLangcode(?Request $request = NULL);
+
+  /**
+   * Notifies the plugin that the language code it returned has been accepted.
+   *
+   * @param \Drupal\Core\Language\LanguageInterface $language
+   *   The accepted language.
+   */
+  public function persist(LanguageInterface $language);
+
+}

web/core/modules/language/src/LanguageNegotiationMethodManager.php

@@ -0,0 +1,33 @@
+<?php
+
+namespace Drupal\language;
+
+use Drupal\Core\Cache\CacheBackendInterface;
+use Drupal\Core\Extension\ModuleHandlerInterface;
+use Drupal\Core\Plugin\DefaultPluginManager;
+use Drupal\language\Attribute\LanguageNegotiation;
+
+/**
+ * Manages language negotiation methods.
+ */
+class LanguageNegotiationMethodManager extends DefaultPluginManager {
+
+  /**
+   * Constructs a new LanguageNegotiationMethodManager object.
+   *
+   * @param \Traversable $namespaces
+   *   An object that implements \Traversable which contains the root paths
+   *   keyed by the corresponding namespace to look for plugin implementations.
+   * @param \Drupal\Core\Cache\CacheBackendInterface $cache_backend
+   *   An object that implements CacheBackendInterface
+   * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
+   *   An object that implements ModuleHandlerInterface
+   */
+  public function __construct(\Traversable $namespaces, CacheBackendInterface $cache_backend, ModuleHandlerInterface $module_handler) {
+    parent::__construct('Plugin/LanguageNegotiation', $namespaces, $module_handler, 'Drupal\language\LanguageNegotiationMethodInterface', LanguageNegotiation::class, 'Drupal\language\Annotation\LanguageNegotiation');
+    $this->cacheBackend = $cache_backend;
+    $this->setCacheBackend($cache_backend, 'language_negotiation_plugins');
+    $this->alterInfo('language_negotiation_info');
+  }
+
+}

web/core/modules/language/src/LanguageNegotiator.php

@@ -0,0 +1,375 @@
+<?php
+
+namespace Drupal\language;
+
+use Drupal\Component\Plugin\Exception\PluginNotFoundException;
+use Drupal\Component\Plugin\PluginManagerInterface;
+use Drupal\Core\Config\ConfigFactoryInterface;
+use Drupal\Core\Logger\LoggerChannelTrait;
+use Drupal\Core\Session\AccountInterface;
+use Drupal\Core\Site\Settings;
+use Drupal\language\Plugin\LanguageNegotiation\LanguageNegotiationUI;
+use Symfony\Component\HttpFoundation\RequestStack;
+
+/**
+ * Class responsible for performing language negotiation.
+ */
+class LanguageNegotiator implements LanguageNegotiatorInterface {
+  use LoggerChannelTrait;
+
+  /**
+   * The language negotiation method plugin manager.
+   *
+   * @var \Drupal\Component\Plugin\PluginManagerInterface
+   */
+  protected $negotiatorManager;
+
+  /**
+   * The language manager.
+   *
+   * @var \Drupal\language\ConfigurableLanguageManagerInterface
+   */
+  protected $languageManager;
+
+  /**
+   * The configuration factory.
+   *
+   * @var \Drupal\Core\Config\ConfigFactoryInterface
+   */
+  protected $configFactory;
+
+  /**
+   * The settings instance.
+   *
+   * @var \Drupal\Core\Site\Settings
+   */
+  protected $settings;
+
+  /**
+   * The request stack object.
+   *
+   * @var \Symfony\Component\HttpFoundation\RequestStack
+   */
+  protected $requestStack;
+
+  /**
+   * The current active user.
+   *
+   * @var \Drupal\Core\Session\AccountInterface
+   */
+  protected $currentUser;
+
+  /**
+   * Local cache for language negotiation method instances.
+   *
+   * @var array
+   */
+  protected $methods;
+
+  /**
+   * An array of language objects keyed by method id.
+   *
+   * @var \Drupal\Core\Language\LanguageInterface[]
+   */
+  protected $negotiatedLanguages = [];
+
+  /**
+   * Constructs a new LanguageNegotiator object.
+   *
+   * @param \Drupal\language\ConfigurableLanguageManagerInterface $language_manager
+   *   The language manager.
+   * @param \Drupal\Component\Plugin\PluginManagerInterface $negotiator_manager
+   *   The language negotiation methods plugin manager.
+   * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
+   *   The configuration factory.
+   * @param \Drupal\Core\Site\Settings $settings
+   *   The settings instance.
+   * @param \Symfony\Component\HttpFoundation\RequestStack $requestStack
+   *   The request stack service.
+   */
+  public function __construct(ConfigurableLanguageManagerInterface $language_manager, PluginManagerInterface $negotiator_manager, ConfigFactoryInterface $config_factory, Settings $settings, RequestStack $requestStack) {
+    $this->languageManager = $language_manager;
+    $this->negotiatorManager = $negotiator_manager;
+    $this->configFactory = $config_factory;
+    $this->settings = $settings;
+    $this->requestStack = $requestStack;
+  }
+
+  /**
+   * Initializes the injected language manager with the negotiator.
+   *
+   * This should be called right after instantiating the negotiator to make it
+   * available to the language manager without introducing a circular
+   * dependency.
+   */
+  public function initLanguageManager() {
+    $this->languageManager->setNegotiator($this);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function reset() {
+    $this->negotiatedLanguages = [];
+    $this->methods = [];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setCurrentUser(AccountInterface $current_user) {
+    $this->currentUser = $current_user;
+    $this->reset();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function initializeType($type) {
+    $language = NULL;
+
+    if ($this->currentUser) {
+      // Execute the language negotiation methods in the order they were set up
+      // and return the first valid language found.
+      foreach ($this->getEnabledNegotiators($type) as $method_id => $info) {
+        if (!isset($this->negotiatedLanguages[$method_id])) {
+          try {
+            $this->negotiatedLanguages[$method_id] = $this->negotiateLanguage($type, $method_id);
+          }
+          catch (PluginNotFoundException $e) {
+            // If a plugin is not found, log the error so user can handle it.
+            $this->getLogger('language')->error($e->getMessage());
+          }
+        }
+
+        // Since objects are references, we need to return a clone to prevent
+        // the language negotiation method cache from being unintentionally
+        // altered. The same methods might be used with different language types
+        // based on configuration.
+        $language = !empty($this->negotiatedLanguages[$method_id]) ? clone($this->negotiatedLanguages[$method_id]) : NULL;
+
+        if ($language) {
+          $this->getNegotiationMethodInstance($method_id)->persist($language);
+          break;
+        }
+      }
+    }
+
+    if (!$language) {
+      // If no other language was found use the default one.
+      $language = $this->languageManager->getDefaultLanguage();
+      $method_id = static::METHOD_ID;
+    }
+
+    return [$method_id => $language];
+  }
+
+  /**
+   * Gets enabled detection methods for the provided language type.
+   *
+   * @param string $type
+   *   The language type.
+   *
+   * @return array
+   *   An array of enabled detection methods for the provided language type.
+   */
+  protected function getEnabledNegotiators($type) {
+    return $this->configFactory->get('language.types')->get('negotiation.' . $type . '.enabled') ?: [];
+  }
+
+  /**
+   * Performs language negotiation using the specified negotiation method.
+   *
+   * @param string $type
+   *   The language type to be initialized.
+   * @param string $method_id
+   *   The string identifier of the language negotiation method to use to detect
+   *   language.
+   *
+   * @return \Drupal\Core\Language\LanguageInterface|null
+   *   Negotiated language object for given type and method, FALSE otherwise.
+   */
+  protected function negotiateLanguage($type, $method_id) {
+    $langcode = NULL;
+    $method = $this->negotiatorManager->getDefinition($method_id);
+
+    if (!isset($method['types']) || in_array($type, $method['types'])) {
+      $langcode = $this->getNegotiationMethodInstance($method_id)->getLangcode($this->requestStack->getCurrentRequest());
+    }
+
+    $languages = $this->languageManager->getLanguages();
+    return $languages[$langcode] ?? NULL;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getNegotiationMethods($type = NULL) {
+    $definitions = $this->negotiatorManager->getDefinitions();
+    if (isset($type)) {
+      $enabled_methods = $this->getEnabledNegotiators($type);
+      $definitions = array_intersect_key($definitions, $enabled_methods);
+    }
+    return $definitions;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getNegotiationMethodInstance($method_id) {
+    if (!isset($this->methods[$method_id])) {
+      $instance = $this->negotiatorManager->createInstance($method_id, []);
+      $instance->setLanguageManager($this->languageManager);
+      $instance->setConfig($this->configFactory);
+      $instance->setCurrentUser($this->currentUser);
+      $this->methods[$method_id] = $instance;
+    }
+    return $this->methods[$method_id];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getPrimaryNegotiationMethod($type) {
+    $enabled_methods = $this->getEnabledNegotiators($type);
+    return empty($enabled_methods) ? LanguageNegotiatorInterface::METHOD_ID : key($enabled_methods);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function isNegotiationMethodEnabled($method_id, $type = NULL) {
+    $enabled = FALSE;
+    $language_types = !empty($type) ? [$type] : $this->languageManager->getLanguageTypes();
+
+    foreach ($language_types as $type) {
+      $enabled_methods = $this->getEnabledNegotiators($type);
+      if (isset($enabled_methods[$method_id])) {
+        $enabled = TRUE;
+        break;
+      }
+    }
+
+    return $enabled;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function saveConfiguration($type, $enabled_methods) {
+    // As configurable language types might have changed, we reset the cache.
+    $this->languageManager->reset();
+    $definitions = $this->getNegotiationMethods();
+    $default_types = $this->languageManager->getLanguageTypes();
+
+    // Ensure that the weights are integers.
+    $enabled_methods = array_map('intval', $enabled_methods);
+
+    // Order the language negotiation method list by weight.
+    asort($enabled_methods);
+    foreach ($enabled_methods as $method_id => $weight) {
+      if (isset($definitions[$method_id])) {
+        $method = $definitions[$method_id];
+        // If the language negotiation method does not express any preference
+        // about types, make it available for any configurable type.
+        $types = array_flip(!empty($method['types']) ? $method['types'] : $default_types);
+        // Check whether the method is defined and has the right type.
+        if (!isset($types[$type])) {
+          unset($enabled_methods[$method_id]);
+        }
+      }
+      else {
+        unset($enabled_methods[$method_id]);
+      }
+    }
+    $this->configFactory->getEditable('language.types')->set('negotiation.' . $type . '.enabled', $enabled_methods)->save(TRUE);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function purgeConfiguration() {
+    // Ensure that we are getting the defined language negotiation information.
+    // An invocation of \Drupal\Core\Extension\ModuleInstaller::install() or
+    // \Drupal\Core\Extension\ModuleInstaller::uninstall() could invalidate the
+    // cached information.
+    $this->negotiatorManager->clearCachedDefinitions();
+    $this->languageManager->reset();
+    foreach ($this->languageManager->getDefinedLanguageTypesInfo() as $type => $info) {
+      $this->saveConfiguration($type, $this->getEnabledNegotiators($type));
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function updateConfiguration(array $types) {
+    // Ensure that we are getting the defined language negotiation information.
+    // An invocation of \Drupal\Core\Extension\ModuleInstaller::install() or
+    // \Drupal\Core\Extension\ModuleInstaller::uninstall() could invalidate the
+    // cached information.
+    $this->negotiatorManager->clearCachedDefinitions();
+    $this->languageManager->reset();
+
+    $language_types = [];
+    $language_types_info = $this->languageManager->getDefinedLanguageTypesInfo();
+    $method_definitions = $this->getNegotiationMethods();
+
+    foreach ($language_types_info as $type => $info) {
+      $configurable = in_array($type, $types);
+
+      // The default language negotiation settings, if available, are stored in
+      // $info['fixed'].
+      $has_default_settings = !empty($info['fixed']);
+      // Check whether the language type is unlocked. Only the status of
+      // unlocked language types can be toggled between configurable and
+      // non-configurable.
+      if (empty($info['locked'])) {
+        if (!$configurable && !$has_default_settings) {
+          // If we have an unlocked non-configurable language type without
+          // default language negotiation settings, we use the values
+          // negotiated for the interface language which, should always be
+          // available.
+          $method_weights = [LanguageNegotiationUI::METHOD_ID];
+          $method_weights = array_flip($method_weights);
+          $this->saveConfiguration($type, $method_weights);
+        }
+      }
+      else {
+        // The language type is locked. Locked language types with default
+        // settings are always considered non-configurable. In turn if default
+        // settings are missing, the language type is always considered
+        // configurable.
+
+        // If the language type is locked we can just store its default language
+        // negotiation settings if it has some, since it is not configurable.
+        if ($has_default_settings) {
+          $method_weights = [];
+          // Default settings are in $info['fixed'].
+
+          foreach ($info['fixed'] as $weight => $method_id) {
+            if (isset($method_definitions[$method_id])) {
+              $method_weights[$method_id] = $weight;
+            }
+          }
+          $this->saveConfiguration($type, $method_weights);
+        }
+        else {
+          // It was missing default settings, so force it to be configurable.
+          $configurable = TRUE;
+        }
+      }
+
+      // Accumulate information for each language type so it can be saved later.
+      $language_types[$type] = $configurable;
+    }
+
+    // Store the language type configuration.
+    $config = [
+      'configurable' => array_keys(array_filter($language_types)),
+      'all' => array_keys($language_types),
+    ];
+    $this->languageManager->saveLanguageTypesConfiguration($config);
+  }
+
+}

web/core/modules/language/src/LanguageNegotiatorInterface.php

@@ -0,0 +1,208 @@
+<?php
+
+namespace Drupal\language;
+
+use Drupal\Core\Session\AccountInterface;
+
+/**
+ * Common interface for language negotiation services.
+ *
+ * The language negotiation API is based on two major concepts:
+ * - Language types: types of translatable data (the types of data that a user
+ *   can view or request).
+ * - Language negotiation methods: responsible for determining which language to
+ *   use to present a particular piece of data to the user.
+ * Both language types and language negotiation methods are customizable.
+ *
+ * Drupal defines three built-in language types:
+ * - Interface language: The page's main language, used to present translated
+ *   user interface elements such as titles, labels, help text, and messages.
+ * - Content language: The language used to present content that is available
+ *   in more than one language.
+ * - URL language: The language associated with URLs. When generating a URL,
+ *   this value will be used for URL's as a default if no explicit preference is
+ *   provided.
+ * Modules can define additional language types through
+ * hook_language_types_info(), and alter existing language type definitions
+ * through hook_language_types_info_alter().
+ *
+ * Language types may be configurable or fixed. The language negotiation
+ * methods associated with a configurable language type can be explicitly
+ * set through the user interface. A fixed language type has predetermined
+ * (module-defined) language negotiation settings and, thus, does not appear in
+ * the configuration page. Here is a code snippet that makes the content
+ * language (which by default inherits the interface language's values)
+ * configurable:
+ * @code
+ * function my_module_language_types_info_alter(&$language_types) {
+ *   unset($language_types[LanguageInterface::TYPE_CONTENT]['fixed']);
+ * }
+ * @endcode
+ *
+ * The locked configuration property prevents one language type from being
+ * switched from customized to not customized, and vice versa.
+ * @see \Drupal\language\LanguageNegotiator::updateConfiguration()
+ *
+ * Every language type can have a different set of language negotiation methods
+ * assigned to it. Different language types often share the same language
+ * negotiation settings, but they can have independent settings if needed. If
+ * two language types are configured the same way, their language switcher
+ * configuration will be functionally identical and the same settings will act
+ * on both language types.
+ *
+ * Drupal defines the following built-in language negotiation methods:
+ * - URL: Determine the language from the URL (path prefix or domain).
+ * - Session: Determine the language from a request/session parameter.
+ * - User: Follow the user's language preference.
+ * - User admin language: Identify admin language from the user preferences.
+ * - Browser: Determine the language from the browser's language settings.
+ * - Selected language: Use the default site language.
+ * Language negotiation methods are simple plugin classes that implement a
+ * particular logic to return a language code. For instance, the URL method
+ * searches for a valid path prefix or domain name in the current request URL.
+ * If a language negotiation method does not return a valid language code, the
+ * next method associated with the language type (based on method weight) is
+ * invoked.
+ *
+ * Modules can define additional language negotiation methods by simply provide
+ * the related plugins, and alter existing methods through
+ * hook_language_negotiation_info_alter(). Here is an example snippet that lets
+ * path prefixes be ignored for administrative paths:
+ * @code
+ * function my_module_language_negotiation_info_alter(&$negotiation_info) {
+ *   // Replace the original plugin with our own implementation.
+ *   $method_id = \Drupal\language\Plugin\LanguageNegotiation\LanguageNegotiationUrl::METHOD_ID;
+ *   $negotiation_info[$method_id]['class'] = 'Drupal\my_module\Plugin\LanguageNegotiation\MyLanguageNegotiationUrl';
+ * }
+ *
+ * class MyLanguageNegotiationUrl extends LanguageNegotiationUrl {
+ *   public function getLangcode(Request $request = NULL) {
+ *     if ($request) {
+ *       // Use the original URL language negotiation method to get a valid
+ *       // language code.
+ *       $langcode = parent::getLangcode($request);
+ *
+ *       // If we are on an administrative path, override with the default
+ *       language.
+ *       if ($request->query->has('q') && strtok($request->query->get('q'), '/') == 'admin') {
+ *         return $this->languageManager->getDefaultLanguage()->getId();
+ *       }
+ *       return $langcode;
+ *     }
+ *   }
+ * }
+ * @endcode
+ *
+ * For more information, see
+ * @link https://www.drupal.org/node/1497272 Language Negotiation API @endlink
+ */
+interface LanguageNegotiatorInterface {
+
+  /**
+   * The language negotiation method id for the language negotiator itself.
+   */
+  const METHOD_ID = 'language-default';
+
+  /**
+   * Resets the negotiated languages and the method instances.
+   */
+  public function reset();
+
+  /**
+   * Sets the current active user and resets all language types.
+   *
+   * @param \Drupal\Core\Session\AccountInterface $current_user
+   *   The current active user.
+   */
+  public function setCurrentUser(AccountInterface $current_user);
+
+  /**
+   * Initializes the specified language type.
+   *
+   * @param string $type
+   *   The language type to be initialized.
+   *
+   * @return \Drupal\Core\Language\LanguageInterface[]
+   *   Returns an array containing a single language keyed by the language
+   *   negotiation method ID used to determine the language of the specified
+   *   type. If negotiation is not possible the default language is returned.
+   */
+  public function initializeType($type);
+
+  /**
+   * Returns the language negotiation methods enabled for a language type.
+   *
+   * @param string $type
+   *   (optional) The language type. If no type is specified all the method
+   *   definitions are returned.
+   *
+   * @return array[]
+   *   An array of language negotiation method definitions keyed by method id.
+   */
+  public function getNegotiationMethods($type = NULL);
+
+  /**
+   * Returns an instance of the specified language negotiation method.
+   *
+   * @param string $method_id
+   *   The method identifier.
+   *
+   * @return \Drupal\language\LanguageNegotiationMethodInterface
+   */
+  public function getNegotiationMethodInstance($method_id);
+
+  /**
+   * Returns the ID of the language type's primary language negotiation method.
+   *
+   * @param string $type
+   *   The language type.
+   *
+   * @return string
+   *   The identifier of the primary language negotiation method for the given
+   *   language type, or the default method if none exists.
+   */
+  public function getPrimaryNegotiationMethod($type);
+
+  /**
+   * Checks whether a language negotiation method is enabled for a language type.
+   *
+   * @param string $method_id
+   *   The language negotiation method ID.
+   * @param string $type
+   *   (optional) The language type. If none is passed, all the configurable
+   *   language types will be inspected.
+   *
+   * @return bool
+   *   TRUE if the method is enabled for at least one of the given language
+   *   types, or FALSE otherwise.
+   */
+  public function isNegotiationMethodEnabled($method_id, $type = NULL);
+
+  /**
+   * Saves a list of language negotiation methods for a language type.
+   *
+   * @param string $type
+   *   The language type.
+   * @param int[] $enabled_methods
+   *   An array of language negotiation method weights keyed by method ID.
+   */
+  public function saveConfiguration($type, $enabled_methods);
+
+  /**
+   * Resave the configuration to purge missing negotiation methods.
+   */
+  public function purgeConfiguration();
+
+  /**
+   * Updates the configuration based on the given language types.
+   *
+   * Stores the list of the language types along with information about their
+   * configurable state. Stores the default settings if the language type is
+   * not configurable.
+   *
+   * @param string[] $types
+   *   An array of configurable language types.
+   */
+  public function updateConfiguration(array $types);
+
+}

web/core/modules/language/src/LanguageServiceProvider.php

@@ -0,0 +1,103 @@
+<?php
+
+namespace Drupal\language;
+
+use Drupal\Core\Config\BootstrapConfigStorageFactory;
+use Drupal\Core\DependencyInjection\ContainerBuilder;
+use Drupal\Core\DependencyInjection\ServiceProviderBase;
+use Drupal\Core\Language\LanguageInterface;
+use Symfony\Component\DependencyInjection\Reference;
+
+/**
+ * Overrides the language_manager service to point to language's module one.
+ */
+class LanguageServiceProvider extends ServiceProviderBase {
+
+  const CONFIG_PREFIX = 'language.entity.';
+
+  /**
+   * {@inheritdoc}
+   */
+  public function register(ContainerBuilder $container) {
+    // The following services are needed only on multilingual sites.
+    if ($this->isMultilingual()) {
+      $container->register('language_request_subscriber', 'Drupal\language\EventSubscriber\LanguageRequestSubscriber')
+        ->addTag('event_subscriber')
+        ->addArgument(new Reference('language_manager'))
+        ->addArgument(new Reference('language_negotiator'))
+        ->addArgument(new Reference('string_translation'))
+        ->addArgument(new Reference('current_user'));
+
+      $container->register('path_processor_language', 'Drupal\language\HttpKernel\PathProcessorLanguage')
+        ->addTag('path_processor_inbound', ['priority' => 300])
+        ->addTag('path_processor_outbound', ['priority' => 100])
+        ->addArgument(new Reference('config.factory'))
+        ->addArgument(new Reference('language_manager'))
+        ->addArgument(new Reference('language_negotiator'))
+        ->addArgument(new Reference('current_user'))
+        ->addArgument(new Reference('language.config_subscriber'))
+        ->addMethodCall('initConfigSubscriber');
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function alter(ContainerBuilder $container) {
+    $definition = $container->getDefinition('language_manager');
+    $definition->setClass('Drupal\language\ConfigurableLanguageManager')
+      ->addArgument(new Reference('config.factory'))
+      ->addArgument(new Reference('module_handler'))
+      ->addArgument(new Reference('language.config_factory_override'))
+      ->addArgument(new Reference('request_stack'));
+    if ($default_language_values = $this->getDefaultLanguageValues()) {
+      $container->setParameter('language.default_values', $default_language_values);
+    }
+
+  }
+
+  /**
+   * Checks whether the site is multilingual.
+   *
+   * @return bool
+   *   TRUE if the site is multilingual, FALSE otherwise.
+   */
+  protected function isMultilingual() {
+    // Assign the prefix to a local variable so it can be used in an anonymous
+    // function.
+    $prefix = static::CONFIG_PREFIX;
+    // @todo Try to swap out for config.storage to take advantage of database
+    //   and caching. This might prove difficult as this is called before the
+    //   container has finished building.
+    $config_storage = BootstrapConfigStorageFactory::get();
+    $config_ids = array_filter($config_storage->listAll($prefix), function ($config_id) use ($prefix) {
+      return $config_id != $prefix . LanguageInterface::LANGCODE_NOT_SPECIFIED && $config_id != $prefix . LanguageInterface::LANGCODE_NOT_APPLICABLE;
+    });
+    return count($config_ids) > 1;
+  }
+
+  /**
+   * Gets the default language values.
+   *
+   * @return array|bool
+   *   Returns the default language values for the language configured in
+   *   system.site:default_langcode if the corresponding configuration entity
+   *   exists, otherwise FALSE.
+   */
+  protected function getDefaultLanguageValues() {
+    $config_storage = BootstrapConfigStorageFactory::get();
+    $system = $config_storage->read('system.site');
+    // In Kernel tests it's possible this code is called before system.site
+    // exists. In such cases behave as though the corresponding language
+    // configuration entity does not exist.
+    if ($system === FALSE) {
+      return FALSE;
+    }
+    $default_language = $config_storage->read(static::CONFIG_PREFIX . $system['default_langcode']);
+    if (is_array($default_language)) {
+      return $default_language;
+    }
+    return FALSE;
+  }
+
+}

web/core/modules/language/src/LanguageSwitcherInterface.php

@@ -0,0 +1,28 @@
+<?php
+
+namespace Drupal\language;
+
+use Drupal\Core\Url;
+use Symfony\Component\HttpFoundation\Request;
+
+/**
+ * Interface for language switcher classes.
+ */
+interface LanguageSwitcherInterface {
+
+  /**
+   * Returns language switch links.
+   *
+   * @param \Symfony\Component\HttpFoundation\Request $request
+   *   The current request.
+   * @param string $type
+   *   The language type.
+   * @param \Drupal\Core\Url $url
+   *   The URL the switch links will be relative to.
+   *
+   * @return array
+   *   An array of link arrays keyed by language code.
+   */
+  public function getLanguageSwitchLinks(Request $request, $type, Url $url);
+
+}

web/core/modules/language/src/Plugin/Block/LanguageBlock.php

@@ -0,0 +1,128 @@
+<?php
+
+namespace Drupal\language\Plugin\Block;
+
+use Drupal\Core\Access\AccessResult;
+use Drupal\Core\Block\Attribute\Block;
+use Drupal\Core\Block\BlockBase;
+use Drupal\Core\Path\PathMatcherInterface;
+use Drupal\Core\Session\AccountInterface;
+use Drupal\Core\Language\LanguageManagerInterface;
+use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
+use Drupal\Core\StringTranslation\TranslatableMarkup;
+use Drupal\Core\Url;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+use Drupal\language\Plugin\Derivative\LanguageBlock as LanguageBlockDeriver;
+
+/**
+ * Provides a 'Language switcher' block.
+ */
+#[Block(
+  id: "language_block",
+  admin_label: new TranslatableMarkup("Language switcher"),
+  category: new TranslatableMarkup("System"),
+  deriver: LanguageBlockDeriver::class
+)]
+class LanguageBlock extends BlockBase implements ContainerFactoryPluginInterface {
+
+  /**
+   * The language manager.
+   *
+   * @var \Drupal\Core\Language\LanguageManagerInterface
+   */
+  protected $languageManager;
+
+  /**
+   * The path matcher.
+   *
+   * @var \Drupal\Core\Path\PathMatcherInterface
+   */
+  protected $pathMatcher;
+
+  /**
+   * Constructs a LanguageBlock object.
+   *
+   * @param array $configuration
+   *   A configuration array containing information about the plugin instance.
+   * @param string $plugin_id
+   *   The plugin ID for the plugin instance.
+   * @param mixed $plugin_definition
+   *   The plugin implementation definition.
+   * @param \Drupal\Core\Language\LanguageManagerInterface $language_manager
+   *   The language manager.
+   * @param \Drupal\Core\Path\PathMatcherInterface $path_matcher
+   *   The path matcher.
+   */
+  public function __construct(array $configuration, $plugin_id, $plugin_definition, LanguageManagerInterface $language_manager, PathMatcherInterface $path_matcher) {
+    parent::__construct($configuration, $plugin_id, $plugin_definition);
+    $this->languageManager = $language_manager;
+    $this->pathMatcher = $path_matcher;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
+    return new static(
+      $configuration,
+      $plugin_id,
+      $plugin_definition,
+      $container->get('language_manager'),
+      $container->get('path.matcher')
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function blockAccess(AccountInterface $account) {
+    $access = $this->languageManager->isMultilingual() ? AccessResult::allowed() : AccessResult::forbidden();
+    return $access->addCacheTags(['config:configurable_language_list']);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function build() {
+    $build = [];
+    $type = $this->getDerivativeId();
+    $route_match = \Drupal::routeMatch();
+    // If there is no route match, for example when creating blocks on 404 pages
+    // for logged-in users with big_pipe enabled using the front page instead.
+    if ($this->pathMatcher->isFrontPage() || !$route_match->getRouteObject()) {
+      // We are skipping the route match on both 404 and front page.
+      // Example: If on front page, there is no route match like when creating
+      // blocks on 404 pages for logged-in users with big_pipe enabled, use the
+      // front page.
+      $url = Url::fromRoute('<front>');
+    }
+    else {
+      $url = Url::fromRouteMatch($route_match);
+    }
+    $links = $this->languageManager->getLanguageSwitchLinks($type, $url);
+
+    if (isset($links->links)) {
+      $build = [
+        '#theme' => 'links__language_block',
+        '#links' => $links->links,
+        '#attributes' => [
+          'class' => [
+            "language-switcher-{$links->method_id}",
+          ],
+        ],
+        '#set_active_class' => TRUE,
+      ];
+    }
+    return $build;
+  }
+
+  /**
+   * {@inheritdoc}
+   *
+   * @todo Make cacheable in https://www.drupal.org/node/2232375.
+   */
+  public function getCacheMaxAge() {
+    return 0;
+  }
+
+}

web/core/modules/language/src/Plugin/Condition/Language.php

@@ -0,0 +1,155 @@
+<?php
+
+namespace Drupal\language\Plugin\Condition;
+
+use Drupal\Core\Condition\Attribute\Condition;
+use Drupal\Core\Condition\ConditionPluginBase;
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Language\LanguageInterface;
+use Drupal\Core\Language\LanguageManagerInterface;
+use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
+use Drupal\Core\Plugin\Context\ContextDefinition;
+use Drupal\Core\StringTranslation\TranslatableMarkup;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Provides a 'Language' condition.
+ */
+#[Condition(
+  id: "language",
+  label: new TranslatableMarkup("Language"),
+  context_definitions: [
+    "language" => new ContextDefinition(
+      data_type: "language",
+      label: new TranslatableMarkup("Language"),
+    ),
+  ]
+)]
+class Language extends ConditionPluginBase implements ContainerFactoryPluginInterface {
+
+  /**
+   * The Language manager.
+   *
+   * @var \Drupal\Core\Language\LanguageManagerInterface
+   */
+  protected $languageManager;
+
+  /**
+   * Creates a new Language instance.
+   *
+   * @param \Drupal\Core\Language\LanguageManagerInterface $language_manager
+   *   The language manager.
+   * @param array $configuration
+   *   The plugin configuration, i.e. an array with configuration values keyed
+   *   by configuration option name. The special key 'context' may be used to
+   *   initialize the defined contexts by setting it to an array of context
+   *   values keyed by context names.
+   * @param string $plugin_id
+   *   The plugin ID for the plugin instance.
+   * @param mixed $plugin_definition
+   *   The plugin implementation definition.
+   */
+  public function __construct(LanguageManagerInterface $language_manager, array $configuration, $plugin_id, $plugin_definition) {
+    parent::__construct($configuration, $plugin_id, $plugin_definition);
+    $this->languageManager = $language_manager;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
+    return new static(
+      $container->get('language_manager'),
+      $configuration,
+      $plugin_id,
+      $plugin_definition
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildConfigurationForm(array $form, FormStateInterface $form_state) {
+    if ($this->languageManager->isMultilingual()) {
+      // Fetch languages.
+      $languages = $this->languageManager->getLanguages();
+      $langcodes_options = [];
+      foreach ($languages as $language) {
+        $langcodes_options[$language->getId()] = $language->getName();
+      }
+      $form['langcodes'] = [
+        '#type' => 'checkboxes',
+        '#title' => $this->t('Language selection'),
+        '#default_value' => $this->configuration['langcodes'],
+        '#options' => $langcodes_options,
+        '#description' => $this->t('Select languages to enforce. If none are selected, all languages will be allowed.'),
+      ];
+    }
+    else {
+      $form['langcodes'] = [
+        '#type' => 'value',
+        '#default_value' => $this->configuration['langcodes'],
+      ];
+    }
+    return parent::buildConfigurationForm($form, $form_state);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function submitConfigurationForm(array &$form, FormStateInterface $form_state) {
+    $this->configuration['langcodes'] = array_filter($form_state->getValue('langcodes'));
+    parent::submitConfigurationForm($form, $form_state);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function summary() {
+    $language_list = $this->languageManager->getLanguages(LanguageInterface::STATE_ALL);
+    $selected = $this->configuration['langcodes'];
+    // Reduce the language list to an array of language names.
+    $language_names = array_reduce($language_list, function ($result, $item) use ($selected) {
+      // If the current item of the $language_list array is one of the selected
+      // languages, add it to the $results array.
+      if (!empty($selected[$item->getId()])) {
+        $result[$item->getId()] = $item->getName();
+      }
+      return $result;
+    }, []);
+
+    // If we have more than one language selected, separate them by commas.
+    if (count($this->configuration['langcodes']) > 1) {
+      $languages = implode(', ', $language_names);
+    }
+    else {
+      // If we have just one language just grab the only present value.
+      $languages = array_pop($language_names);
+    }
+    if (!empty($this->configuration['negate'])) {
+      return $this->t('The language is not @languages.', ['@languages' => $languages]);
+    }
+    return $this->t('The language is @languages.', ['@languages' => $languages]);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function evaluate() {
+    if (empty($this->configuration['langcodes']) && !$this->isNegated()) {
+      return TRUE;
+    }
+
+    $language = $this->getContextValue('language');
+    // Language visibility settings.
+    return !empty($this->configuration['langcodes'][$language->getId()]);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function defaultConfiguration() {
+    return ['langcodes' => []] + parent::defaultConfiguration();
+  }
+
+}

web/core/modules/language/src/Plugin/Derivative/LanguageBlock.php

@@ -0,0 +1,39 @@
+<?php
+
+namespace Drupal\language\Plugin\Derivative;
+
+use Drupal\Component\Plugin\Derivative\DeriverBase;
+use Drupal\Core\StringTranslation\StringTranslationTrait;
+use Drupal\language\ConfigurableLanguageManagerInterface;
+
+/**
+ * Provides language switcher block plugin definitions for all languages.
+ */
+class LanguageBlock extends DeriverBase {
+
+  use StringTranslationTrait;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getDerivativeDefinitions($base_plugin_definition) {
+    $language_manager = \Drupal::languageManager();
+
+    if ($language_manager instanceof ConfigurableLanguageManagerInterface) {
+      $info = $language_manager->getDefinedLanguageTypesInfo();
+      $configurable_types = $language_manager->getLanguageTypes();
+      foreach ($configurable_types as $type) {
+        $this->derivatives[$type] = $base_plugin_definition;
+        $this->derivatives[$type]['admin_label'] = $this->t('Language switcher (@type)', ['@type' => $info[$type]['name']]);
+      }
+      // If there is just one configurable type then change the title of the
+      // block.
+      if (count($configurable_types) == 1) {
+        $this->derivatives[reset($configurable_types)]['admin_label'] = $this->t('Language switcher');
+      }
+    }
+
+    return parent::getDerivativeDefinitions($base_plugin_definition);
+  }
+
+}

web/core/modules/language/src/Plugin/LanguageNegotiation/LanguageNegotiationBrowser.php

@@ -0,0 +1,68 @@
+<?php
+
+namespace Drupal\language\Plugin\LanguageNegotiation;
+
+use Drupal\Component\Utility\UserAgent;
+use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
+use Drupal\Core\StringTranslation\TranslatableMarkup;
+use Drupal\language\Attribute\LanguageNegotiation;
+use Drupal\language\LanguageNegotiationMethodBase;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+use Symfony\Component\HttpFoundation\Request;
+
+/**
+ * Class for identifying language from the browser Accept-language HTTP header.
+ */
+#[LanguageNegotiation(
+  id: LanguageNegotiationBrowser::METHOD_ID,
+  name: new TranslatableMarkup('Browser'),
+  weight: -2,
+  description: new TranslatableMarkup("Language from the browser's language settings."),
+  config_route_name: 'language.negotiation_browser'
+)]
+class LanguageNegotiationBrowser extends LanguageNegotiationMethodBase implements ContainerFactoryPluginInterface {
+
+  /**
+   * The language negotiation method id.
+   */
+  const METHOD_ID = 'language-browser';
+
+  /**
+   * The page cache disabling policy.
+   *
+   * @var \Drupal\Core\PageCache\ResponsePolicy\KillSwitch
+   */
+  protected $pageCacheKillSwitch;
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
+    $instance = new static();
+    $instance->pageCacheKillSwitch = $container->get('page_cache_kill_switch');
+    return $instance;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getLangcode(?Request $request = NULL) {
+    $langcode = NULL;
+
+    if ($this->languageManager && $request && $request->server->get('HTTP_ACCEPT_LANGUAGE')) {
+      $http_accept_language = $request->server->get('HTTP_ACCEPT_LANGUAGE');
+      $langcodes = array_keys($this->languageManager->getLanguages());
+      $mappings = $this->config->get('language.mappings')->get('map');
+      $langcode = UserAgent::getBestMatchingLangcode($http_accept_language, $langcodes, $mappings);
+    }
+
+    // Internal page cache with multiple languages and browser negotiation
+    // could lead to wrong cached sites. Therefore disabling the internal page
+    // cache.
+    // @todo Solve more elegantly in https://www.drupal.org/node/2430335.
+    $this->pageCacheKillSwitch->trigger();
+
+    return $langcode;
+  }
+
+}

web/core/modules/language/src/Plugin/LanguageNegotiation/LanguageNegotiationContentEntity.php

@@ -0,0 +1,282 @@
+<?php
+
+namespace Drupal\language\Plugin\LanguageNegotiation;
+
+use Drupal\Core\Entity\ContentEntityInterface;
+use Drupal\Core\Entity\EntityTypeManagerInterface;
+use Drupal\Core\Language\LanguageInterface;
+use Drupal\Core\PathProcessor\OutboundPathProcessorInterface;
+use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
+use Drupal\Core\Render\BubbleableMetadata;
+use Drupal\Core\StringTranslation\TranslatableMarkup;
+use Drupal\Core\Url;
+use Drupal\language\Attribute\LanguageNegotiation;
+use Drupal\language\LanguageNegotiationMethodBase;
+use Drupal\language\LanguageSwitcherInterface;
+use Drupal\Core\Routing\RouteObjectInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+use Symfony\Component\HttpFoundation\Request;
+use Symfony\Component\Routing\Route;
+
+/**
+ * Class for identifying the content translation language.
+ */
+#[LanguageNegotiation(
+  id: LanguageNegotiationContentEntity::METHOD_ID,
+  name: new TranslatableMarkup('Content language'),
+  types: [LanguageInterface::TYPE_CONTENT],
+  weight: -9,
+  description: new TranslatableMarkup("Determines the content language from the request parameter named 'language_content_entity'.")
+)]
+class LanguageNegotiationContentEntity extends LanguageNegotiationMethodBase implements OutboundPathProcessorInterface, LanguageSwitcherInterface, ContainerFactoryPluginInterface {
+
+  /**
+   * The language negotiation method ID.
+   */
+  const METHOD_ID = 'language-content-entity';
+
+  /**
+   * The query string parameter.
+   */
+  const QUERY_PARAMETER = 'language_content_entity';
+
+  /**
+   * A list of all the link paths of enabled content entities.
+   *
+   * @var array
+   */
+  protected $contentEntityPaths;
+
+  /**
+   * Static cache for the language negotiation order check.
+   *
+   * @see \Drupal\language\Plugin\LanguageNegotiation\LanguageNegotiationContentEntity::hasLowerLanguageNegotiationWeight()
+   *
+   * @var bool
+   */
+  protected $hasLowerLanguageNegotiationWeightResult;
+
+  /**
+   * Static cache of outbound route paths per request.
+   *
+   * @var \SplObjectStorage
+   */
+  protected $paths;
+
+  /**
+   * The entity type manager.
+   *
+   * @var \Drupal\Core\Entity\EntityTypeManagerInterface
+   */
+  protected $entityTypeManager;
+
+  /**
+   * Constructs a new LanguageNegotiationContentEntity instance.
+   *
+   * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager
+   *   The entity type manager.
+   */
+  public function __construct(EntityTypeManagerInterface $entity_type_manager) {
+    $this->entityTypeManager = $entity_type_manager;
+    $this->paths = new \SplObjectStorage();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
+    return new static($container->get('entity_type.manager'));
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getLangcode(?Request $request = NULL) {
+    if ($request === NULL || $this->languageManager === NULL) {
+      return NULL;
+    }
+
+    $langcode = $request->query->get(static::QUERY_PARAMETER);
+
+    $language_enabled = array_key_exists($langcode, $this->languageManager->getLanguages());
+    return $language_enabled ? $langcode : NULL;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function processOutbound($path, &$options = [], ?Request $request = NULL, ?BubbleableMetadata $bubbleable_metadata = NULL) {
+    // If appropriate, process outbound to add a query parameter to the URL and
+    // remove the language option, so that URL negotiator does not rewrite the
+    // URL.
+
+    // First, check if processing conditions are met.
+    if (!($request && !empty($options['route']) && $this->hasLowerLanguageNegotiationWeight() && $this->meetsContentEntityRoutesCondition($options['route'], $request))) {
+      return $path;
+    }
+
+    if (isset($options['language']) || $langcode = $this->getLangcode($request)) {
+      // If the language option is set, unset it, so that the URL language
+      // negotiator does not rewrite the URL.
+      if (isset($options['language'])) {
+        $langcode = $options['language']->getId();
+        unset($options['language']);
+      }
+
+      if (!isset($options['query'][static::QUERY_PARAMETER])) {
+        $options['query'][static::QUERY_PARAMETER] = $langcode;
+      }
+
+      if ($bubbleable_metadata) {
+        // Cached URLs that have been processed by this outbound path
+        // processor must be:
+        $bubbleable_metadata
+          // - varied by the content language query parameter.
+          ->addCacheContexts(['url.query_args:' . static::QUERY_PARAMETER]);
+      }
+    }
+
+    return $path;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getLanguageSwitchLinks(Request $request, $type, Url $url) {
+    $links = [];
+    $query = [];
+    parse_str($request->getQueryString() ?? '', $query);
+
+    foreach ($this->languageManager->getNativeLanguages() as $language) {
+      $langcode = $language->getId();
+      $query[static::QUERY_PARAMETER] = $langcode;
+      $links[$langcode] = [
+        'url' => $url,
+        'title' => $language->getName(),
+        'attributes' => ['class' => ['language-link']],
+        'query' => $query,
+      ];
+    }
+
+    return $links;
+  }
+
+  /**
+   * Determines if content entity language negotiator has higher priority.
+   *
+   * The content entity language negotiator having higher priority than the URL
+   * language negotiator, is a criteria in
+   * \Drupal\language\Plugin\LanguageNegotiation\LanguageNegotiationContentEntity::processOutbound().
+   *
+   * @return bool
+   *   TRUE if the content entity language negotiator has higher priority than
+   *   the URL language negotiator, FALSE otherwise.
+   */
+  protected function hasLowerLanguageNegotiationWeight() {
+    if (!isset($this->hasLowerLanguageNegotiationWeightResult)) {
+      // Only run if the LanguageNegotiationContentEntity outbound function is
+      // being executed before the outbound function of LanguageNegotiationUrl.
+      $content_method_weights = $this->config->get('language.types')->get('negotiation.language_content.enabled') ?: [];
+
+      // Check if the content language is configured to be dependent on the
+      // URL negotiator directly or indirectly over the interface negotiator.
+      if (isset($content_method_weights[LanguageNegotiationUrl::METHOD_ID]) && ($content_method_weights[static::METHOD_ID] > $content_method_weights[LanguageNegotiationUrl::METHOD_ID])) {
+        $this->hasLowerLanguageNegotiationWeightResult = FALSE;
+      }
+      else {
+        $check_interface_method = FALSE;
+        if (isset($content_method_weights[LanguageNegotiationUI::METHOD_ID])) {
+          $interface_method_weights = $this->config->get('language.types')->get('negotiation.language_interface.enabled') ?: [];
+          $check_interface_method = isset($interface_method_weights[LanguageNegotiationUrl::METHOD_ID]);
+        }
+        if ($check_interface_method) {
+          $max_weight = $content_method_weights[LanguageNegotiationUI::METHOD_ID];
+          $max_weight = isset($content_method_weights[LanguageNegotiationUrl::METHOD_ID]) ? max($max_weight, $content_method_weights[LanguageNegotiationUrl::METHOD_ID]) : $max_weight;
+        }
+        else {
+          $max_weight = $content_method_weights[LanguageNegotiationUrl::METHOD_ID] ?? PHP_INT_MAX;
+        }
+
+        $this->hasLowerLanguageNegotiationWeightResult = $content_method_weights[static::METHOD_ID] < $max_weight;
+      }
+    }
+
+    return $this->hasLowerLanguageNegotiationWeightResult;
+  }
+
+  /**
+   * Determines if content entity route condition is met.
+   *
+   * Requirements: currently being on a content entity route and processing
+   * outbound URL pointing to the same content entity.
+   *
+   * @param \Symfony\Component\Routing\Route $outbound_route
+   *   The route object for the current outbound URL being processed.
+   * @param \Symfony\Component\HttpFoundation\Request $request
+   *   The HttpRequest object representing the current request.
+   *
+   * @return bool
+   *   TRUE if the content entity route condition is met, FALSE otherwise.
+   */
+  protected function meetsContentEntityRoutesCondition(Route $outbound_route, Request $request) {
+    $outbound_path_pattern = $outbound_route->getPath();
+    $storage = $this->paths[$request] ?? [];
+    if (!isset($storage[$outbound_path_pattern])) {
+      $storage[$outbound_path_pattern] = FALSE;
+
+      // Check if the outbound route points to the current entity.
+      if ($content_entity_type_id_for_current_route = $this->getContentEntityTypeIdForCurrentRequest($request)) {
+        if (!empty($this->getContentEntityPaths()[$outbound_path_pattern]) && $content_entity_type_id_for_current_route == $this->getContentEntityPaths()[$outbound_path_pattern]) {
+          $storage[$outbound_path_pattern] = TRUE;
+        }
+      }
+
+      $this->paths[$request] = $storage;
+    }
+
+    return $storage[$outbound_path_pattern];
+  }
+
+  /**
+   * Returns the content entity type ID from the current request for the route.
+   *
+   * @param \Symfony\Component\HttpFoundation\Request $request
+   *   The HttpRequest object representing the current request.
+   *
+   * @return string
+   *   The entity type ID for the route from the request.
+   */
+  protected function getContentEntityTypeIdForCurrentRequest(Request $request) {
+    $content_entity_type_id_for_current_route = '';
+
+    if ($current_route = $request->attributes->get(RouteObjectInterface::ROUTE_OBJECT)) {
+      $current_route_path = $current_route->getPath();
+      $content_entity_type_id_for_current_route = $this->getContentEntityPaths()[$current_route_path] ?? '';
+    }
+
+    return $content_entity_type_id_for_current_route;
+  }
+
+  /**
+   * Returns the paths for the link templates of all content entities.
+   *
+   * @return array
+   *   An array of all content entity type IDs, keyed by the corresponding link
+   *   template paths.
+   */
+  protected function getContentEntityPaths() {
+    if (!isset($this->contentEntityPaths)) {
+      $this->contentEntityPaths = [];
+      $entity_types = $this->entityTypeManager->getDefinitions();
+      foreach ($entity_types as $entity_type_id => $entity_type) {
+        if ($entity_type->entityClassImplements(ContentEntityInterface::class)) {
+          $entity_paths = array_fill_keys($entity_type->getLinkTemplates(), $entity_type_id);
+          $this->contentEntityPaths = array_merge($this->contentEntityPaths, $entity_paths);
+        }
+      }
+    }
+
+    return $this->contentEntityPaths;
+  }
+
+}

web/core/modules/language/src/Plugin/LanguageNegotiation/LanguageNegotiationSelected.php

@@ -0,0 +1,40 @@
+<?php
+
+namespace Drupal\language\Plugin\LanguageNegotiation;
+
+use Drupal\Core\StringTranslation\TranslatableMarkup;
+use Drupal\language\Attribute\LanguageNegotiation;
+use Drupal\language\LanguageNegotiationMethodBase;
+use Symfony\Component\HttpFoundation\Request;
+
+/**
+ * Class for identifying language from a selected language.
+ */
+#[LanguageNegotiation(
+  id: LanguageNegotiationSelected::METHOD_ID,
+  name: new TranslatableMarkup('Selected language'),
+  weight: 12,
+  description: new TranslatableMarkup("Language based on a selected language."),
+  config_route_name: 'language.negotiation_selected'
+)]
+class LanguageNegotiationSelected extends LanguageNegotiationMethodBase {
+
+  /**
+   * The language negotiation method id.
+   */
+  const METHOD_ID = 'language-selected';
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getLangcode(?Request $request = NULL) {
+    $langcode = NULL;
+
+    if ($this->languageManager) {
+      $langcode = $this->config->get('language.negotiation')->get('selected_langcode');
+    }
+
+    return $langcode;
+  }
+
+}

web/core/modules/language/src/Plugin/LanguageNegotiation/LanguageNegotiationSession.php

@@ -0,0 +1,189 @@
+<?php
+
+namespace Drupal\language\Plugin\LanguageNegotiation;
+
+use Drupal\Core\Language\LanguageInterface;
+use Drupal\Core\PathProcessor\OutboundPathProcessorInterface;
+use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
+use Drupal\Core\Render\BubbleableMetadata;
+use Drupal\Core\StringTranslation\TranslatableMarkup;
+use Drupal\Core\Url;
+use Drupal\language\Attribute\LanguageNegotiation;
+use Drupal\language\LanguageNegotiationMethodBase;
+use Drupal\language\LanguageSwitcherInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+use Symfony\Component\HttpFoundation\Request;
+use Symfony\Component\HttpFoundation\RequestStack;
+
+/**
+ * Identify language from a request/session parameter.
+ */
+#[LanguageNegotiation(
+  id: LanguageNegotiationSession::METHOD_ID,
+  name: new TranslatableMarkup('Session'),
+  weight: -6,
+  description: new TranslatableMarkup("Language from a request/session parameter."),
+  config_route_name: 'language.negotiation_session'
+)]
+class LanguageNegotiationSession extends LanguageNegotiationMethodBase implements OutboundPathProcessorInterface, LanguageSwitcherInterface, ContainerFactoryPluginInterface {
+
+  /**
+   * Flag used to determine whether query rewriting is active.
+   *
+   * @var bool
+   */
+  protected $queryRewrite;
+
+  /**
+   * The query parameter name to rewrite.
+   *
+   * @var string
+   */
+  protected $queryParam;
+
+  /**
+   * The query parameter value to be set.
+   *
+   * @var string
+   */
+  protected $queryValue;
+
+  /**
+   * The request stack.
+   *
+   * @var \Symfony\Component\HttpFoundation\RequestStack
+   */
+  protected $requestStack;
+
+  /**
+   * The language negotiation method id.
+   */
+  const METHOD_ID = 'language-session';
+
+  /**
+   * Constructs a LanguageNegotiationSession object.
+   *
+   * @param \Symfony\Component\HttpFoundation\RequestStack $request_stack
+   *   The request stack.
+   */
+  public function __construct(RequestStack $request_stack) {
+    $this->requestStack = $request_stack;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
+    return new static(
+      $container->get('request_stack')
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getLangcode(?Request $request = NULL) {
+    $config = $this->config->get('language.negotiation')->get('session');
+    if (($param = $config['parameter']) && $request) {
+      if ($request->query->has($param)) {
+        return $request->query->get($param);
+      }
+      if ($request->getSession()->has($param)) {
+        return $request->getSession()->get($param);
+      }
+    }
+    return NULL;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function persist(LanguageInterface $language) {
+    // We need to update the session parameter with the request value only if we
+    // have an authenticated user.
+    $langcode = $language->getId();
+    if ($langcode && $this->languageManager) {
+      $languages = $this->languageManager->getLanguages();
+      if ($this->currentUser->isAuthenticated() && isset($languages[$langcode])) {
+        $config = $this->config->get('language.negotiation')->get('session');
+        $this->requestStack->getCurrentRequest()->getSession()->set($config['parameter'], $langcode);
+      }
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function processOutbound($path, &$options = [], ?Request $request = NULL, ?BubbleableMetadata $bubbleable_metadata = NULL) {
+    if ($request) {
+      // The following values are not supposed to change during a single page
+      // request processing.
+      if (!isset($this->queryRewrite)) {
+        if ($this->currentUser->isAnonymous()) {
+          $languages = $this->languageManager->getLanguages();
+          $config = $this->config->get('language.negotiation')->get('session');
+          $this->queryParam = $config['parameter'];
+          $this->queryValue = $request->query->has($this->queryParam) ? $request->query->get($this->queryParam) : NULL;
+          $this->queryRewrite = isset($languages[$this->queryValue]);
+        }
+        else {
+          $this->queryRewrite = FALSE;
+        }
+      }
+
+      // If the user is anonymous, the user language negotiation method is
+      // enabled, and the corresponding option has been set, we must preserve
+      // any explicit user language preference even with cookies disabled.
+      if ($this->queryRewrite) {
+        if (!isset($options['query'][$this->queryParam])) {
+          $options['query'][$this->queryParam] = $this->queryValue;
+        }
+        if ($bubbleable_metadata) {
+          // Cached URLs that have been processed by this outbound path
+          // processor must be:
+          $bubbleable_metadata
+            // - invalidated when the language negotiation config changes, since
+            //   another query parameter may be used to determine the language.
+            ->addCacheTags($this->config->get('language.negotiation')->getCacheTags())
+            // - varied by the configured query parameter.
+            ->addCacheContexts(['url.query_args:' . $this->queryParam]);
+        }
+      }
+    }
+    return $path;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getLanguageSwitchLinks(Request $request, $type, Url $url) {
+    $links = [];
+    $query = [];
+    parse_str($request->getQueryString() ?? '', $query);
+    $config = $this->config->get('language.negotiation')->get('session');
+    $param = $config['parameter'];
+    $language_query = $request->getSession()->has($param) ? $request->getSession()->get($param) : $this->languageManager->getCurrentLanguage($type)->getId();
+
+    foreach ($this->languageManager->getNativeLanguages() as $language) {
+      $langcode = $language->getId();
+      $links[$langcode] = [
+        // We need to clone the $url object to avoid using the same one for all
+        // links. When the links are rendered, options are set on the $url
+        // object, so if we use the same one, they would be set for all links.
+        'url' => clone $url,
+        'title' => $language->getName(),
+        'attributes' => ['class' => ['language-link']],
+        'query' => $query,
+      ];
+      if ($language_query != $langcode) {
+        $links[$langcode]['query'][$param] = $langcode;
+      }
+      else {
+        $links[$langcode]['attributes']['class'][] = 'session-active';
+      }
+    }
+
+    return $links;
+  }
+
+}

web/core/modules/language/src/Plugin/LanguageNegotiation/LanguageNegotiationUI.php

@@ -0,0 +1,35 @@
+<?php
+
+namespace Drupal\language\Plugin\LanguageNegotiation;
+
+use Drupal\Core\Language\LanguageInterface;
+use Drupal\Core\StringTranslation\TranslatableMarkup;
+use Drupal\language\Attribute\LanguageNegotiation;
+use Drupal\language\LanguageNegotiationMethodBase;
+use Symfony\Component\HttpFoundation\Request;
+
+/**
+ * Identifies the language from the interface text language selected for page.
+ */
+#[LanguageNegotiation(
+  id: LanguageNegotiationUI::METHOD_ID,
+  name: new TranslatableMarkup('Interface'),
+  types: [LanguageInterface::TYPE_CONTENT],
+  weight: 9,
+  description: new TranslatableMarkup("Use the detected interface language.")
+)]
+class LanguageNegotiationUI extends LanguageNegotiationMethodBase {
+
+  /**
+   * The language negotiation method id.
+   */
+  const METHOD_ID = 'language-interface';
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getLangcode(?Request $request = NULL) {
+    return $this->languageManager ? $this->languageManager->getCurrentLanguage()->getId() : NULL;
+  }
+
+}

web/core/modules/language/src/Plugin/LanguageNegotiation/LanguageNegotiationUrl.php

@@ -0,0 +1,219 @@
+<?php
+
+namespace Drupal\language\Plugin\LanguageNegotiation;
+
+use Drupal\Core\Language\LanguageInterface;
+use Drupal\Core\PathProcessor\InboundPathProcessorInterface;
+use Drupal\Core\PathProcessor\OutboundPathProcessorInterface;
+use Drupal\Core\Render\BubbleableMetadata;
+use Drupal\Core\StringTranslation\TranslatableMarkup;
+use Drupal\Core\Url;
+use Drupal\language\Attribute\LanguageNegotiation;
+use Drupal\language\LanguageNegotiationMethodBase;
+use Drupal\language\LanguageSwitcherInterface;
+use Symfony\Component\HttpFoundation\Request;
+
+/**
+ * Class for identifying language via URL prefix or domain.
+ */
+#[LanguageNegotiation(
+  id: LanguageNegotiationUrl::METHOD_ID,
+  name: new TranslatableMarkup('URL'),
+  types: [LanguageInterface::TYPE_INTERFACE,
+    LanguageInterface::TYPE_CONTENT,
+    LanguageInterface::TYPE_URL,
+  ],
+  weight: -8,
+  description: new TranslatableMarkup("Language from the URL (Path prefix or domain)."),
+  config_route_name: 'language.negotiation_url'
+)]
+class LanguageNegotiationUrl extends LanguageNegotiationMethodBase implements InboundPathProcessorInterface, OutboundPathProcessorInterface, LanguageSwitcherInterface {
+
+  /**
+   * The language negotiation method id.
+   */
+  const METHOD_ID = 'language-url';
+
+  /**
+   * URL language negotiation: use the path prefix as URL language indicator.
+   */
+  const CONFIG_PATH_PREFIX = 'path_prefix';
+
+  /**
+   * URL language negotiation: use the domain as URL language indicator.
+   */
+  const CONFIG_DOMAIN = 'domain';
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getLangcode(?Request $request = NULL) {
+    $langcode = NULL;
+
+    if ($request && $this->languageManager) {
+      $languages = $this->languageManager->getLanguages();
+      $config = $this->config->get('language.negotiation')->get('url');
+
+      switch ($config['source']) {
+        case LanguageNegotiationUrl::CONFIG_PATH_PREFIX:
+          $request_path = urldecode(trim($request->getPathInfo(), '/'));
+          $path_args = explode('/', $request_path);
+          $prefix = array_shift($path_args);
+
+          // Search prefix within added languages.
+          $negotiated_language = FALSE;
+          foreach ($languages as $language) {
+            if (isset($config['prefixes'][$language->getId()]) && $config['prefixes'][$language->getId()] == $prefix) {
+              $negotiated_language = $language;
+              break;
+            }
+          }
+
+          if ($negotiated_language) {
+            $langcode = $negotiated_language->getId();
+          }
+          break;
+
+        case LanguageNegotiationUrl::CONFIG_DOMAIN:
+          // Get only the host, not the port.
+          $http_host = $request->getHost();
+          foreach ($languages as $language) {
+            // Skip the check if the language doesn't have a domain.
+            if (!empty($config['domains'][$language->getId()])) {
+              // Ensure that there is exactly one protocol in the URL when
+              // checking the hostname.
+              $host = 'http://' . str_replace(['http://', 'https://'], '', $config['domains'][$language->getId()]);
+              $host = parse_url($host, PHP_URL_HOST);
+              if ($http_host == $host) {
+                $langcode = $language->getId();
+                break;
+              }
+            }
+          }
+          break;
+      }
+    }
+
+    return $langcode;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function processInbound($path, Request $request) {
+    $config = $this->config->get('language.negotiation')->get('url');
+
+    if ($config['source'] == LanguageNegotiationUrl::CONFIG_PATH_PREFIX) {
+      $parts = explode('/', trim($path, '/'));
+      $prefix = array_shift($parts);
+
+      // Search prefix within added languages.
+      foreach ($this->languageManager->getLanguages() as $language) {
+        if (isset($config['prefixes'][$language->getId()]) && $config['prefixes'][$language->getId()] == $prefix) {
+          // Rebuild $path with the language removed.
+          $path = '/' . implode('/', $parts);
+          break;
+        }
+      }
+    }
+
+    return $path;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function processOutbound($path, &$options = [], ?Request $request = NULL, ?BubbleableMetadata $bubbleable_metadata = NULL) {
+    $url_scheme = 'http';
+    $port = 80;
+    if ($request) {
+      $url_scheme = $request->getScheme();
+      $port = $request->getPort();
+    }
+    $languages = array_flip(array_keys($this->languageManager->getLanguages()));
+    // Language can be passed as an option, or we go for current URL language.
+    if (!isset($options['language']) || ($options['language'] instanceof LanguageInterface && $options['language']->getId() == LanguageInterface::LANGCODE_NOT_SPECIFIED)) {
+      $language_url = $this->languageManager->getCurrentLanguage(LanguageInterface::TYPE_URL);
+      $options['language'] = $language_url;
+    }
+    // We allow only added languages here.
+    elseif (!is_object($options['language']) || !isset($languages[$options['language']->getId()])) {
+      return $path;
+    }
+    $config = $this->config->get('language.negotiation')->get('url');
+    if ($config['source'] == LanguageNegotiationUrl::CONFIG_PATH_PREFIX) {
+      if (is_object($options['language']) && !empty($config['prefixes'][$options['language']->getId()])) {
+        $options['prefix'] = $config['prefixes'][$options['language']->getId()] . '/';
+        if ($bubbleable_metadata) {
+          $bubbleable_metadata->addCacheContexts(['languages:' . LanguageInterface::TYPE_URL]);
+        }
+      }
+    }
+    elseif ($config['source'] == LanguageNegotiationUrl::CONFIG_DOMAIN) {
+      if (is_object($options['language']) && !empty($config['domains'][$options['language']->getId()])) {
+
+        // Save the original base URL. If it contains a port, we need to
+        // retain it below.
+        if (!empty($options['base_url'])) {
+          // The colon in the URL scheme messes up the port checking below.
+          $normalized_base_url = str_replace(['https://', 'http://'], '', $options['base_url']);
+        }
+
+        // Ask for an absolute URL with our modified base URL.
+        $options['absolute'] = TRUE;
+        $options['base_url'] = $url_scheme . '://' . $config['domains'][$options['language']->getId()];
+
+        // In case either the original base URL or the HTTP host contains a
+        // port, retain it.
+        if (isset($normalized_base_url) && str_contains($normalized_base_url, ':')) {
+          [, $port] = explode(':', $normalized_base_url);
+          $options['base_url'] .= ':' . $port;
+        }
+        elseif (($url_scheme == 'http' && $port != 80) || ($url_scheme == 'https' && $port != 443)) {
+          $options['base_url'] .= ':' . $port;
+        }
+
+        if (isset($options['https'])) {
+          if ($options['https'] === TRUE) {
+            $options['base_url'] = str_replace('http://', 'https://', $options['base_url']);
+          }
+          elseif ($options['https'] === FALSE) {
+            $options['base_url'] = str_replace('https://', 'http://', $options['base_url']);
+          }
+        }
+
+        // Add Drupal's subfolder from the base_path if there is one.
+        $options['base_url'] .= rtrim(base_path(), '/');
+        if ($bubbleable_metadata) {
+          $bubbleable_metadata->addCacheContexts(['languages:' . LanguageInterface::TYPE_URL, 'url.site']);
+        }
+      }
+    }
+    return $path;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getLanguageSwitchLinks(Request $request, $type, Url $url) {
+    $links = [];
+    $query = [];
+    parse_str($request->getQueryString() ?? '', $query);
+
+    foreach ($this->languageManager->getNativeLanguages() as $language) {
+      $links[$language->getId()] = [
+        // We need to clone the $url object to avoid using the same one for all
+        // links. When the links are rendered, options are set on the $url
+        // object, so if we use the same one, they would be set for all links.
+        'url' => clone $url,
+        'title' => $language->getName(),
+        'language' => $language,
+        'attributes' => ['class' => ['language-link']],
+        'query' => $query,
+      ];
+    }
+
+    return $links;
+  }
+
+}

web/core/modules/language/src/Plugin/LanguageNegotiation/LanguageNegotiationUrlFallback.php

@@ -0,0 +1,72 @@
+<?php
+
+namespace Drupal\language\Plugin\LanguageNegotiation;
+
+use Drupal\Core\Language\LanguageInterface;
+use Drupal\Core\StringTranslation\TranslatableMarkup;
+use Drupal\language\Attribute\LanguageNegotiation;
+use Drupal\language\LanguageNegotiationMethodBase;
+use Symfony\Component\HttpFoundation\Request;
+
+/**
+ * Determines the language to be assigned to URLs when none is detected.
+ *
+ * The language negotiation process has a fallback chain that ends with the
+ * default language negotiation method. Each built-in language type has a
+ * separate initialization:
+ * - Interface language, which is the only configurable one, always gets a valid
+ *   value. If no request-specific language is detected, the default language
+ *   will be used.
+ * - Content language merely inherits the interface language by default.
+ * - URL language is detected from the requested URL and will be used to rewrite
+ *   URLs appearing in the page being rendered. If no language can be detected,
+ *   there are two possibilities:
+ *   - If the default language has no configured path prefix or domain, then the
+ *     default language is used. This guarantees that (missing) URL prefixes are
+ *     preserved when navigating through the site.
+ *   - If the default language has a configured path prefix or domain, a
+ *     requested URL having an empty prefix or domain is an anomaly that must be
+ *     fixed. This is done by introducing a prefix or domain in the rendered
+ *     page matching the detected interface language.
+ */
+#[LanguageNegotiation(
+  id: LanguageNegotiationUrlFallback::METHOD_ID,
+  name: new TranslatableMarkup('URL fallback'),
+  types: [LanguageInterface::TYPE_URL],
+  weight: 8,
+  description: new TranslatableMarkup('Use an already detected language for URLs if none is found.'),
+)]
+class LanguageNegotiationUrlFallback extends LanguageNegotiationMethodBase {
+
+  /**
+   * The language negotiation method id.
+   */
+  const METHOD_ID = 'language-url-fallback';
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getLangcode(?Request $request = NULL) {
+    $langcode = NULL;
+
+    if ($this->languageManager) {
+      $default = $this->languageManager->getDefaultLanguage();
+      $config = $this->config->get('language.negotiation')->get('url');
+      $prefix = ($config['source'] == LanguageNegotiationUrl::CONFIG_PATH_PREFIX);
+
+      // If the default language is not configured to convey language
+      // information, a missing URL language information indicates that URL
+      // language should be the default one, otherwise we fall back to an
+      // already detected language.
+      if (($prefix && empty($config['prefixes'][$default->getId()])) || (!$prefix && empty($config['domains'][$default->getId()]))) {
+        $langcode = $default->getId();
+      }
+      else {
+        $langcode = $this->languageManager->getCurrentLanguage()->getId();
+      }
+    }
+
+    return $langcode;
+  }
+
+}

web/core/modules/language/src/Plugin/migrate/destination/DefaultLangcode.php

@@ -0,0 +1,34 @@
+<?php
+
+namespace Drupal\language\Plugin\migrate\destination;
+
+use Drupal\language\Entity\ConfigurableLanguage;
+use Drupal\migrate\Attribute\MigrateDestination;
+use Drupal\migrate\MigrateException;
+use Drupal\migrate\Plugin\migrate\destination\Config;
+use Drupal\migrate\Row;
+
+/**
+ * Provides a destination plugin for the default langcode config.
+ */
+#[MigrateDestination('default_langcode')]
+class DefaultLangcode extends Config {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function import(Row $row, array $old_destination_id_values = []) {
+    $destination = $row->getDestination();
+    $langcode = $destination['default_langcode'];
+
+    // Check if the language exists.
+    if (ConfigurableLanguage::load($langcode) === NULL) {
+      throw new MigrateException("The language '$langcode' does not exist on this site.");
+    }
+
+    $this->config->set('default_langcode', $destination['default_langcode']);
+    $this->config->save();
+    return [$this->config->getName()];
+  }
+
+}

web/core/modules/language/src/Plugin/migrate/process/ContentTranslationEnabledSetting.php

@@ -0,0 +1,63 @@
+<?php
+
+namespace Drupal\language\Plugin\migrate\process;
+
+use Drupal\migrate\Attribute\MigrateProcess;
+use Drupal\migrate\MigrateException;
+use Drupal\migrate\MigrateExecutableInterface;
+use Drupal\migrate\ProcessPluginBase;
+use Drupal\migrate\Row;
+
+/**
+ * Determines the content translation setting.
+ *
+ * The source value is an indexed array of three values:
+ * - The language content type, e.g. '1'
+ * - The entity_translation_entity_types, an array of entity types.
+ * - An entity type used with entity translation, e.g. comment.
+ */
+#[MigrateProcess('content_translation_enabled_setting')]
+class ContentTranslationEnabledSetting extends ProcessPluginBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
+    if (!is_array($value)) {
+      throw new MigrateException('Input should be an array');
+    }
+
+    [$language_content_type, $entity_translation_entity_types, $entity_type] = $value;
+
+    switch ($language_content_type) {
+      // In the case of being 0, it will be skipped. We are not actually setting
+      // a null value.
+      case 0;
+        $setting = NULL;
+        break;
+
+      case 1:
+        $setting = FALSE;
+        break;
+
+      case 2:
+        $setting = FALSE;
+        break;
+
+      case 4:
+        // If entity translation is enabled return the status of comment
+        // translations.
+        $setting = FALSE;
+        if (!empty($entity_translation_entity_types[$entity_type])) {
+          $setting = TRUE;
+        }
+        break;
+
+      default:
+        $setting = NULL;
+        break;
+    }
+    return $setting;
+  }
+
+}

web/core/modules/language/src/Plugin/migrate/process/LanguageDomains.php

@@ -0,0 +1,44 @@
+<?php
+
+namespace Drupal\language\Plugin\migrate\process;
+
+use Drupal\migrate\Attribute\MigrateProcess;
+use Drupal\migrate\MigrateExecutableInterface;
+use Drupal\migrate\Plugin\migrate\process\ArrayBuild;
+use Drupal\migrate\Row;
+
+/**
+ * This plugin makes sure that no domain is empty if domain negotiation is used.
+ */
+#[MigrateProcess(
+  id: "language_domains",
+  handle_multiples: TRUE,
+)]
+class LanguageDomains extends ArrayBuild {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
+    if ($row->getSourceProperty('domain_negotiation_used')) {
+      global $base_url;
+
+      foreach ($value as $old_key => $old_value) {
+        if (empty($old_value['domain'])) {
+          // The default language domain might be empty.
+          // If it is, use the current domain.
+          $value[$old_key]['domain'] = parse_url($base_url, PHP_URL_HOST);
+        }
+        else {
+          // Ensure we have a protocol when checking for the hostname.
+          $domain = 'http://' . str_replace(['http://', 'https://'], '', $old_value['domain']);
+          // Only keep the host part of the domain.
+          $value[$old_key]['domain'] = parse_url($domain, PHP_URL_HOST);
+        }
+      }
+    }
+
+    return parent::transform($value, $migrate_executable, $row, $destination_property);
+  }
+
+}

web/core/modules/language/src/Plugin/migrate/process/LanguageNegotiation.php

@@ -0,0 +1,88 @@
+<?php
+
+namespace Drupal\language\Plugin\migrate\process;
+
+use Drupal\migrate\Attribute\MigrateProcess;
+use Drupal\migrate\MigrateException;
+use Drupal\migrate\MigrateExecutableInterface;
+use Drupal\migrate\ProcessPluginBase;
+use Drupal\migrate\Row;
+
+/**
+ * Processes the arrays for the language types' negotiation methods and weights.
+ */
+#[MigrateProcess(
+  id: "language_negotiation",
+  handle_multiples: TRUE,
+)]
+class LanguageNegotiation extends ProcessPluginBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
+    $new_value = [
+      'enabled' => [],
+      'method_weights' => [],
+    ];
+
+    if (!is_array($value)) {
+      throw new MigrateException('The input should be an array');
+    }
+
+    // If no weights are provided, use the keys by flipping the array.
+    if (empty($value[1])) {
+      $new_value['enabled'] = array_flip(array_map([$this, 'mapNewMethods'], array_keys($value[0])));
+      unset($new_value['method_weights']);
+    }
+    else {
+      foreach ($value[1] as $method => $weight) {
+        $new_method = $this->mapNewMethods($method);
+        $new_value['method_weights'][$new_method] = $weight;
+        if (in_array($method, array_keys($value[0]))) {
+          $new_value['enabled'][$new_method] = $weight;
+        }
+      }
+    }
+
+    return $new_value;
+  }
+
+  /**
+   * Maps old negotiation method names to the new ones.
+   *
+   * @param string $value
+   *   The old negotiation method name.
+   *
+   * @return string
+   *   The new negotiation method name.
+   */
+  protected function mapNewMethods($value) {
+    switch ($value) {
+      case 'language-default':
+        return 'language-selected';
+
+      case 'locale-browser':
+        return 'language-browser';
+
+      case 'locale-interface':
+        return 'language-interface';
+
+      case 'locale-session':
+        return 'language-session';
+
+      case 'locale-url':
+        return 'language-url';
+
+      case 'locale-url-fallback':
+        return 'language-url-fallback';
+
+      case 'locale-user':
+        return 'language-user';
+
+      default:
+        return $value;
+    }
+  }
+
+}

web/core/modules/language/src/Plugin/migrate/process/LanguageTypes.php

@@ -0,0 +1,40 @@
+<?php
+
+namespace Drupal\language\Plugin\migrate\process;
+
+use Drupal\migrate\Attribute\MigrateProcess;
+use Drupal\migrate\MigrateException;
+use Drupal\migrate\MigrateExecutableInterface;
+use Drupal\migrate\ProcessPluginBase;
+use Drupal\migrate\Row;
+
+/**
+ * Processes the array for the language types.
+ */
+#[MigrateProcess(
+  id: "language_types",
+  handle_multiples: TRUE,
+)]
+class LanguageTypes extends ProcessPluginBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
+    if (!is_array($value)) {
+      throw new MigrateException('The input should be an array');
+    }
+
+    if (array_key_exists('language', $value)) {
+      $value['language_interface'] = $value['language'];
+      unset($value['language']);
+    }
+
+    if (!empty($this->configuration['filter_configurable'])) {
+      $value = array_filter($value);
+    }
+
+    return array_keys($value);
+  }
+
+}

web/core/modules/language/src/Plugin/migrate/source/Language.php

@@ -0,0 +1,103 @@
+<?php
+
+namespace Drupal\language\Plugin\migrate\source;
+
+use Drupal\migrate\Row;
+use Drupal\migrate_drupal\Plugin\migrate\source\DrupalSqlBase;
+
+/**
+ * Drupal 6/7 language source from database.
+ *
+ * Available configuration keys:
+ * - fetch_all: (optional) If not empty, all source languages are retrieved and
+ *   available as "languages" source property. Each language is an array with
+ *   the same structure as a source row.
+ * - domain_negotiation: (optional) If not empty and domain negotiation is
+ *   enabled in the source database, the "domain_negotiation_used" source
+ *   property is set to TRUE.
+ *
+ * Example:
+ *
+ * @code
+ * plugin: language
+ * fetch_all: true
+ * domain_negotiation: true
+ * @endcode
+ *
+ * In this example, available languages are retrieved from the source database.
+ * Given that fetch_all and domain_negotiation are specified, each row also
+ * contains all languages and the domain negotiation status, if enabled.
+ *
+ * For additional configuration keys, refer to the parent classes.
+ *
+ * @see \Drupal\migrate\Plugin\migrate\source\SqlBase
+ * @see \Drupal\migrate\Plugin\migrate\source\SourcePluginBase
+ *
+ * @MigrateSource(
+ *   id = "language",
+ *   source_module = "locale"
+ * )
+ */
+class Language extends DrupalSqlBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function fields() {
+    return [
+      'language' => $this->t('The language code.'),
+      'name' => $this->t('The English name of the language.'),
+      'native' => $this->t('The native name of the language.'),
+      'direction' => $this->t('The language direction. (0 = LTR, 1 = RTL)'),
+      'enabled' => $this->t('Whether the language is enabled.'),
+      'plurals' => $this->t('Number of plural indexes in this language.'),
+      'formula' => $this->t('PHP formula to get plural indexes.'),
+      'domain' => $this->t('Domain to use for this language.'),
+      'prefix' => $this->t('Path prefix used for this language.'),
+      'weight' => $this->t('The language weight when listed.'),
+      'javascript' => $this->t('Location of the JavaScript translation file.'),
+    ];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getIds() {
+    return [
+      'language' => [
+        'type' => 'string',
+      ],
+    ];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function query() {
+    return $this->select('languages')->fields('languages');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function prepareRow(Row $row) {
+    if (!empty($this->configuration['fetch_all'])) {
+      // Get an array of all languages.
+      $languages = $this->query()->execute()->fetchAll();
+      $row->setSourceProperty('languages', $languages);
+    }
+
+    if (!empty($this->configuration['domain_negotiation'])) {
+      // Check if domain negotiation is used to be able to fill in the default
+      // language domain, which may be empty. In D6, domain negotiation is used
+      // when the 'language_negotiation' variable is set to '3', and in D7, when
+      // the 'locale_language_negotiation_url_part' variable is set to '1'.
+      if ($this->variableGet('language_negotiation', 0) == 3 || $this->variableGet('locale_language_negotiation_url_part', 0) == 1) {
+        $row->setSourceProperty('domain_negotiation_used', TRUE);
+      }
+    }
+
+    return parent::prepareRow($row);
+  }
+
+}

web/core/modules/language/src/Plugin/migrate/source/d6/LanguageContentSettings.php

@@ -0,0 +1,63 @@
+<?php
+
+namespace Drupal\language\Plugin\migrate\source\d6;
+
+use Drupal\migrate\Row;
+use Drupal\migrate_drupal\Plugin\migrate\source\DrupalSqlBase;
+
+/**
+ * Drupal 6 i18n node settings from database.
+ *
+ * For available configuration keys, refer to the parent classes.
+ *
+ * @see \Drupal\migrate\Plugin\migrate\source\SqlBase
+ * @see \Drupal\migrate\Plugin\migrate\source\SourcePluginBas
+ *
+ * @MigrateSource(
+ *   id = "d6_language_content_settings",
+ *   source_module = "locale"
+ * )
+ */
+class LanguageContentSettings extends DrupalSqlBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function query() {
+    return $this->select('node_type', 't')
+      ->fields('t', [
+        'type',
+      ]);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function fields() {
+    $fields = [
+      'type' => $this->t('Type'),
+      'language_content_type' => $this->t('Multilingual support.'),
+      'i18n_lock_node' => $this->t('Lock language.'),
+    ];
+    return $fields;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function prepareRow(Row $row) {
+    $type = $row->getSourceProperty('type');
+    $row->setSourceProperty('language_content_type', $this->variableGet('language_content_type_' . $type, NULL));
+    $row->setSourceProperty('i18n_lock_node', $this->variableGet('i18n_lock_node_' . $type, 0));
+    return parent::prepareRow($row);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getIds() {
+    $ids['type']['type'] = 'string';
+    return $ids;
+  }
+
+}

web/core/modules/language/src/Plugin/migrate/source/d6/LanguageContentSettingsTaxonomyVocabulary.php

@@ -0,0 +1,78 @@
+<?php
+
+namespace Drupal\language\Plugin\migrate\source\d6;
+
+use Drupal\migrate\Row;
+use Drupal\migrate_drupal\Plugin\migrate\source\DrupalSqlBase;
+
+// cspell:ignore localizable
+
+/**
+ * Drupal 6 i18n vocabularies source from database.
+ *
+ * For available configuration keys, refer to the parent classes.
+ *
+ * @see \Drupal\migrate\Plugin\migrate\source\SqlBase
+ * @see \Drupal\migrate\Plugin\migrate\source\SourcePluginBas
+ *
+ * @MigrateSource(
+ *   id = "d6_language_content_settings_taxonomy_vocabulary",
+ *   source_module = "taxonomy"
+ * )
+ */
+class LanguageContentSettingsTaxonomyVocabulary extends DrupalSqlBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function query() {
+    $query = $this->select('vocabulary', 'v')
+      ->fields('v', ['vid']);
+    if ($this->getDatabase()
+      ->schema()
+      ->fieldExists('vocabulary', 'language')) {
+      $query->addField('v', 'language');
+    }
+    return $query;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function fields() {
+    return [
+      'vid' => $this->t('The vocabulary ID.'),
+      'language' => $this->t('The default language for new terms.'),
+      'state' => $this->t('The i18n taxonomy translation setting.'),
+    ];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function prepareRow(Row $row) {
+    // Get the i18n taxonomy translation setting for this vocabulary.
+    // 0 - No multilingual options
+    // 1 - Localizable terms. Run through the localization system.
+    // 2 - Predefined language for a vocabulary and its terms.
+    // 3 - Per-language terms, translatable (referencing terms with different
+    // languages) but not localizable.
+    $i18ntaxonomy_vocabulary = $this->variableGet('i18ntaxonomy_vocabulary', []);
+    $vid = $row->getSourceProperty('vid');
+    $state = 0;
+    if (array_key_exists($vid, $i18ntaxonomy_vocabulary)) {
+      $state = $i18ntaxonomy_vocabulary[$vid];
+    }
+    $row->setSourceProperty('state', $state);
+    return parent::prepareRow($row);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getIds() {
+    $ids['vid']['type'] = 'integer';
+    return $ids;
+  }
+
+}

web/core/modules/language/src/Plugin/migrate/source/d7/LanguageContentSettings.php

@@ -0,0 +1,73 @@
+<?php
+
+namespace Drupal\language\Plugin\migrate\source\d7;
+
+use Drupal\migrate\Row;
+use Drupal\migrate_drupal\Plugin\migrate\source\DrupalSqlBase;
+
+/**
+ * Drupal 7 i18n node settings from database.
+ *
+ * For available configuration keys, refer to the parent classes.
+ *
+ * @see \Drupal\migrate\Plugin\migrate\source\SqlBase
+ * @see \Drupal\migrate\Plugin\migrate\source\SourcePluginBas
+ *
+ * @MigrateSource(
+ *   id = "d7_language_content_settings",
+ *   source_module = "locale"
+ * )
+ */
+class LanguageContentSettings extends DrupalSqlBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function query() {
+    return $this->select('node_type', 't')
+      ->fields('t', [
+        'type',
+      ]);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function fields() {
+    $fields = [
+      'type' => $this->t('Type'),
+      'language_content_type' => $this->t('Multilingual support.'),
+      'i18n_lock_node' => $this->t('Lock language.'),
+    ];
+    return $fields;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function prepareRow(Row $row) {
+    $type = $row->getSourceProperty('type');
+    $row->setSourceProperty('language_content_type', $this->variableGet('language_content_type_' . $type, NULL));
+    $i18n_node_options = $this->variableGet('i18n_node_options_' . $type, NULL);
+    if ($i18n_node_options && in_array('lock', $i18n_node_options)) {
+      $row->setSourceProperty('i18n_lock_node', 1);
+    }
+    else {
+      $row->setSourceProperty('i18n_lock_node', 0);
+    }
+
+    // Get the entity translation entity settings.
+    $entity_translation_entity_types = $this->variableGet('entity_translation_entity_types', NULL);
+    $row->setSourceProperty('entity_translation_entity_types', $entity_translation_entity_types);
+    return parent::prepareRow($row);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getIds() {
+    $ids['type']['type'] = 'string';
+    return $ids;
+  }
+
+}

web/core/modules/language/src/Plugin/migrate/source/d7/LanguageContentSettingsTaxonomyVocabulary.php

@@ -0,0 +1,46 @@
+<?php
+
+namespace Drupal\language\Plugin\migrate\source\d7;
+
+use Drupal\taxonomy\Plugin\migrate\source\d7\Vocabulary;
+
+/**
+ * Drupal 7 i18n vocabularies source from database.
+ *
+ * For available configuration keys, refer to the parent classes.
+ *
+ * @see \Drupal\migrate\Plugin\migrate\source\SqlBase
+ * @see \Drupal\migrate\Plugin\migrate\source\SourcePluginBas
+ *
+ * @MigrateSource(
+ *   id = "d7_language_content_settings_taxonomy_vocabulary",
+ *   source_module = "i18n_taxonomy"
+ * )
+ */
+class LanguageContentSettingsTaxonomyVocabulary extends Vocabulary {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function query() {
+    $query = parent::query();
+    if ($this->getDatabase()
+      ->schema()
+      ->fieldExists('taxonomy_vocabulary', 'i18n_mode')) {
+      $query->addField('v', 'language');
+      $query->addField('v', 'i18n_mode');
+    }
+    return $query;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function fields() {
+    $fields = parent::fields();
+    $fields['language'] = $this->t('i18n language');
+    $fields['i18n_mode'] = $this->t('i18n mode');
+    return $fields;
+  }
+
+}

web/core/modules/language/src/ProxyClass/LanguageConverter.php

@@ -0,0 +1,88 @@
+<?php
+// phpcs:ignoreFile
+
+/**
+ * This file was generated via php core/scripts/generate-proxy-class.php 'Drupal\language\LanguageConverter' "core/modules/language/src".
+ */
+
+namespace Drupal\language\ProxyClass {
+
+    /**
+     * Provides a proxy class for \Drupal\language\LanguageConverter.
+     *
+     * @see \Drupal\Component\ProxyBuilder
+     */
+    class LanguageConverter implements \Drupal\Core\ParamConverter\ParamConverterInterface
+    {
+
+        use \Drupal\Core\DependencyInjection\DependencySerializationTrait;
+
+        /**
+         * The id of the original proxied service.
+         *
+         * @var string
+         */
+        protected $drupalProxyOriginalServiceId;
+
+        /**
+         * The real proxied service, after it was lazy loaded.
+         *
+         * @var \Drupal\language\LanguageConverter
+         */
+        protected $service;
+
+        /**
+         * The service container.
+         *
+         * @var \Symfony\Component\DependencyInjection\ContainerInterface
+         */
+        protected $container;
+
+        /**
+         * Constructs a ProxyClass Drupal proxy object.
+         *
+         * @param \Symfony\Component\DependencyInjection\ContainerInterface $container
+         *   The container.
+         * @param string $drupal_proxy_original_service_id
+         *   The service ID of the original service.
+         */
+        public function __construct(\Symfony\Component\DependencyInjection\ContainerInterface $container, $drupal_proxy_original_service_id)
+        {
+            $this->container = $container;
+            $this->drupalProxyOriginalServiceId = $drupal_proxy_original_service_id;
+        }
+
+        /**
+         * Lazy loads the real service from the container.
+         *
+         * @return object
+         *   Returns the constructed real service.
+         */
+        protected function lazyLoadItself()
+        {
+            if (!isset($this->service)) {
+                $this->service = $this->container->get($this->drupalProxyOriginalServiceId);
+            }
+
+            return $this->service;
+        }
+
+        /**
+         * {@inheritdoc}
+         */
+        public function convert($value, $definition, $name, array $defaults)
+        {
+            return $this->lazyLoadItself()->convert($value, $definition, $name, $defaults);
+        }
+
+        /**
+         * {@inheritdoc}
+         */
+        public function applies($definition, $name, \Symfony\Component\Routing\Route $route)
+        {
+            return $this->lazyLoadItself()->applies($definition, $name, $route);
+        }
+
+    }
+
+}