class UserLangcode

class UserLangcode extends ProcessPluginBase implements ContainerFactoryPluginInterface {
  public function __construct(array $configuration, $plugin_id, $plugin_definition, #[Autowire(service: 'language_manager')] protected LanguageManager $languageManager) {
    parent::__construct($configuration, $plugin_id, $plugin_definition);
  }
  
  /**
   * {@inheritdoc}
   */
  public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
    $this->configuration += [
      'fallback_to_site_default' => TRUE,
    ];
    // If the user's language is empty, then default to English and set the
    // user's preferred_langcode and preferred_admin_langcode to the default
    // language.
    if (empty($value)) {
      return $this->configuration['fallback_to_site_default'] ? $this->languageManager
        ->getDefaultLanguage()
        ->getId() : 'en';
    }
    elseif ($this->languageManager
      ->getLanguage($value) === NULL) {
      return $this->languageManager
        ->getDefaultLanguage()
        ->getId();
    }
    // If the langcode is a valid one, then just return it.
    return $value;
  }

}