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;
    }

}