MailLanguageManager and LanguageNegotiationMail

LanguageNegotiationMail.php

<?php

namespace Drupal\example\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;

#[LanguageNegotiation(
  id: LanguageNegotiationMail::METHOD_ID,
  name: new TranslatableMarkup('Mail'),
  types: [
    LanguageInterface::TYPE_INTERFACE,
    LanguageInterface::TYPE_CONTENT,
    LanguageInterface::TYPE_URL,
  ],
  weight: -50,
  description: new TranslatableMarkup("Language from context (for Mail rendering).")
)]
class LanguageNegotiationMail extends LanguageNegotiationMethodBase {

  /**
   * The language negotiation method id.
   */
  const string METHOD_ID = 'language-mail';

  const string REQUEST_ATTRIBUTE = 'example_module_mail_langcode';

  public function getLangcode(?Request $request = NULL) {
    return $request?->attributes->get($this::REQUEST_ATTRIBUTE);
  }

}

MailLanguageManager.php

<?php

namespace Drupal\example;

use Drupal\example\Plugin\LanguageNegotiation\LanguageNegotiationMail;
use Drupal\Core\Cache\MemoryCache\MemoryCacheInterface;
use Drupal\Core\DependencyInjection\AutowireTrait;
use Drupal\Core\Entity\EntityFieldManagerInterface;
use Drupal\Core\Language\LanguageManagerInterface;
use Drupal\Core\StringTranslation\TranslationInterface;
use Drupal\Core\TypedData\TypedDataManager;
use Drupal\Core\TypedData\TypedDataManagerInterface;
use Drupal\language\EventSubscriber\LanguageRequestSubscriber;
use Drupal\language\LanguageNegotiatorInterface;
use Symfony\Component\DependencyInjection\Attribute\Autowire;

class MailLanguageManager {

  use AutowireTrait;

  public function __construct(
    protected LanguageManagerInterface $languageManager,
    #[Autowire(service: 'string_translation')]
    protected TranslationInterface $stringTranslation,
    protected LanguageNegotiatorInterface $languageNegotiator,
    #[Autowire(service: 'language_request_subscriber')]
    protected LanguageRequestSubscriber $languageRequestSubscriber,
    protected TypedDataManagerInterface $typedDataManager,
    protected EntityFieldManagerInterface $entityFieldManager,
    #[Autowire(service: 'entity.memory_cache')]
    protected MemoryCacheInterface $entityMemoryCache,
  ) {

  }

  public function reset() {
    \Drupal::request()->attributes->remove(LanguageNegotiationMail::REQUEST_ATTRIBUTE);
    $this->languageRequestSubscriber->onContainerInitializeSubrequestFinished();

    // Clear prototype cache in typed data manager.
    // @see https://www.drupal.org/i/2276749
    // @see https://www.drupal.org/i/3221375
    $resetPrototypes = \Closure::bind(
      function (): void {
        $this->prototypes = [];
      },
      $this->typedDataManager,
      $this->typedDataManager
    );

    $resetPrototypes();

    // Rest options_allowed_values, because is not language aware.
    drupal_static_reset('options_allowed_values');

    // Flush cache in EntityFieldManager
    // \Drupal\Core\Entity\EntityFieldManager::getBaseFieldDefinitions has a
    // static cache and it not language aware.
    $this->entityFieldManager->useCaches(FALSE);
    $this->entityFieldManager->useCaches(TRUE);
    $this->entityMemoryCache->deleteAll();
  }

  public function setLanguage($langcode) {
    $this->reset();
    \Drupal::request()->attributes->set(LanguageNegotiationMail::REQUEST_ATTRIBUTE, $langcode);
    $this->languageRequestSubscriber->onContainerInitializeSubrequestFinished();
  }

}