class LanguageFormatter

class LanguageFormatter extends StringFormatter {
  
  /**
   * The language manager.
   *
   * @var \Drupal\Core\Language\LanguageManagerInterface
   */
  protected $languageManager;
  
  /**
   * Constructs a LanguageFormatter instance.
   *
   * @param string $plugin_id
   *   The plugin_id for the formatter.
   * @param mixed $plugin_definition
   *   The plugin implementation definition.
   * @param \Drupal\Core\Field\FieldDefinitionInterface $field_definition
   *   The definition of the field to which the formatter is associated.
   * @param array $settings
   *   The formatter settings.
   * @param string $label
   *   The formatter label display setting.
   * @param string $view_mode
   *   The view mode.
   * @param array $third_party_settings
   *   Any third party settings settings.
   * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager
   *   The entity type manager.
   * @param \Drupal\Core\Language\LanguageManagerInterface $language_manager
   *   The language manager.
   */
  public function __construct($plugin_id, $plugin_definition, FieldDefinitionInterface $field_definition, array $settings, $label, $view_mode, array $third_party_settings, EntityTypeManagerInterface $entity_type_manager, LanguageManagerInterface $language_manager) {
    parent::__construct($plugin_id, $plugin_definition, $field_definition, $settings, $label, $view_mode, $third_party_settings, $entity_type_manager);
    $this->languageManager = $language_manager;
  }
  
  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
    return new static($plugin_id, $plugin_definition, $configuration['field_definition'], $configuration['settings'], $configuration['label'], $configuration['view_mode'], $configuration['third_party_settings'], $container->get('entity_type.manager'), $container->get('language_manager'));
  }
  
  /**
   * {@inheritdoc}
   */
  public static function defaultSettings() {
    $settings = parent::defaultSettings();
    $settings['native_language'] = FALSE;
    return $settings;
  }
  
  /**
   * {@inheritdoc}
   */
  public function settingsForm(array $form, FormStateInterface $form_state) {
    $form = parent::settingsForm($form, $form_state);
    $form['native_language'] = [
      '#title' => $this->t('Display in native language'),
      '#type' => 'checkbox',
      '#default_value' => $this->getSetting('native_language'),
    ];
    return $form;
  }
  
  /**
   * {@inheritdoc}
   */
  public function settingsSummary() {
    $summary = parent::settingsSummary();
    if ($this->getSetting('native_language')) {
      $summary[] = $this->t('Displayed in native language');
    }
    return $summary;
  }
  
  /**
   * {@inheritdoc}
   */
  protected function viewValue(FieldItemInterface $item) {
    // The 'languages' cache context is not necessary because the language is
    // either displayed in its configured form (loaded directly from config
    // storage by LanguageManager::getLanguages()) or in its native language
    // name. That only depends on formatter settings and no language condition.
    $languages = $this->getSetting('native_language') ? $this->languageManager
      ->getNativeLanguages(LanguageInterface::STATE_ALL) : $this->languageManager
      ->getLanguages(LanguageInterface::STATE_ALL);
    return [
      '#plain_text' => $item->language && isset($languages[$item->language
        ->getId()]) ? $languages[$item->language
        ->getId()]
        ->getName() : '',
    ];
  }

}