class UserLangcode

class UserLangcode extends ProcessPluginBase implements ContainerFactoryPluginInterface {
  
  /**
   * The language manager.
   *
   * @var \Drupal\Core\Language\LanguageManager
   */
  protected $languageManager;
  
  /**
   * Constructs a UserLangcode object.
   *
   * @param array $configuration
   *   Plugin configuration.
   * @param string $plugin_id
   *   The plugin ID.
   * @param mixed $plugin_definition
   *   The plugin definition.
   * @param \Drupal\Core\Language\LanguageManager $language_manager
   *   The language manager service.
   */
  public function __construct(array $configuration, $plugin_id, $plugin_definition, LanguageManager $language_manager) {
    parent::__construct($configuration, $plugin_id, $plugin_definition);
    $this->languageManager = $language_manager;
  }
  
  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
    return new static($configuration, $plugin_id, $plugin_definition, $container->get('language_manager'));
  }
  
  /**
   * {@inheritdoc}
   */
  public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
    if (!isset($this->configuration['fallback_to_site_default'])) {
      $this->configuration['fallback_to_site_default'] = TRUE;
    }
    // If the user's language is empty, it means the locale module was not
    // installed, so the user's langcode should be English and the user's
    // preferred_langcode and preferred_admin_langcode should fallback to the
    // default language.
    if (empty($value)) {
      if ($this->configuration['fallback_to_site_default']) {
        return $this->languageManager
          ->getDefaultLanguage()
          ->getId();
      }
      else {
        return 'en';
      }
    }
    elseif ($this->languageManager
      ->getLanguage($value) === NULL) {
      return $this->languageManager
        ->getDefaultLanguage()
        ->getId();
    }
    // If the langcode is a valid one, just return it.
    return $value;
  }

}