class LanguageConfiguration

class LanguageConfiguration extends FormElementBase {

  /**
   * {@inheritdoc}
   */
  public function getInfo() {
    $class = static::class;
    return [
      '#input' => TRUE,
      '#tree' => TRUE,
      '#process' => [
        [
          $class,
          'processLanguageConfiguration',
        ],
      ],
    ];
  }

  /**
   * Process handler for the language_configuration form element.
   */
  public static function processLanguageConfiguration(&$element, FormStateInterface $form_state, &$form) {
    $options = $element['#options'] ?? [];

    // Avoid validation failure since we are moving the '#options' key in the
    // nested 'language' select element.
    unset($element['#options']);

    /** @var \Drupal\language\Entity\ContentLanguageSettings $default_config */
    $default_config = $element['#default_value'];
    $element['langcode'] = [
      '#type' => 'select',
      '#title' => t('Default language'),
      '#options' => $options + static::getDefaultOptions(),
      '#description' => t('Explanation of the language options is found on the <a href=":languages_list_page">languages list page</a>.', [
        ':languages_list_page' => Url::fromRoute('entity.configurable_language.collection')
          ->toString(),
      ]),
      '#default_value' => $default_config != NULL ? $default_config
        ->getDefaultLangcode() : LanguageInterface::LANGCODE_SITE_DEFAULT,
    ];
    $element['language_alterable'] = [
      '#type' => 'checkbox',
      '#title' => t('Show language selector on create and edit pages'),
      '#default_value' => $default_config != NULL ? $default_config
        ->isLanguageAlterable() : FALSE,
    ];

    // Add the entity type and bundle information to the form if they are set.
    // They will be used, in the submit handler, to generate the names of the
    // configuration entities that will store the settings and are a way to uniquely
    // identify the entity.
    $language = $form_state
      ->get('language') ?: [];
    $language += [
      $element['#name'] => [
        'entity_type' => $element['#entity_information']['entity_type'],
        'bundle' => $element['#entity_information']['bundle'],
      ],
    ];
    $form_state
      ->set('language', $language);

    // Do not add the submit callback for the language content settings page,
    // which is handled separately.
    if ($form['#form_id'] != 'language_content_settings_form') {

      // Determine where to attach the language_configuration element submit
      // handler.
      // @todo Form API: Allow form widgets/sections to declare #submit
      //   handlers.
      $submit_name = isset($form['actions']['save_continue']) ? 'save_continue' : 'submit';
      if (isset($form['actions'][$submit_name]['#submit']) && array_search('language_configuration_element_submit', $form['actions'][$submit_name]['#submit']) === FALSE) {
        $form['actions'][$submit_name]['#submit'][] = 'language_configuration_element_submit';
      }
      elseif (array_search('language_configuration_element_submit', $form['#submit']) === FALSE) {
        $form['#submit'][] = 'language_configuration_element_submit';
      }
    }
    return $element;
  }

  /**
   * Returns the default options for the language configuration form element.
   *
   * @return array
   *   An array containing the default options.
   */
  protected static function getDefaultOptions() {
    $language_options = [
      LanguageInterface::LANGCODE_SITE_DEFAULT => t("Site's default language (@language)", [
        '@language' => static::languageManager()
          ->getDefaultLanguage()
          ->getName(),
      ]),
      'current_interface' => t('Interface text language selected for page'),
      'authors_default' => t("Author's preferred language"),
    ];
    $languages = static::languageManager()
      ->getLanguages(LanguageInterface::STATE_ALL);
    foreach ($languages as $langcode => $language) {
      $language_options[$langcode] = $language
        ->isLocked() ? t('- @name -', [
        '@name' => $language
          ->getName(),
      ]) : $language
        ->getName();
    }
    return $language_options;
  }

  /**
   * Wraps the language manager.
   *
   * @return \Drupal\Core\Language\LanguageManagerInterface
   */
  protected static function languageManager() {
    return \Drupal::languageManager();
  }

}