LanguageSelectWidget.php

<?php

namespace Drupal\Core\Field\Plugin\Field\FieldWidget;

use Drupal\Core\Field\FieldItemListInterface;
use Drupal\Core\Field\WidgetBase;
use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Language\LanguageInterface;

/**
 * Plugin implementation of the 'Language' widget.
 *
 * @FieldWidget(
 *   id = "language_select",
 *   label = @Translation("Language select"),
 *   field_types = {
 *     "language"
 *   }
 * )
 */
class LanguageSelectWidget extends WidgetBase {

  /**
   * {@inheritdoc}
   */
  public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
    $element['value'] = $element + array(
      '#type' => 'language_select',
      '#default_value' => $items[$delta]->value,
      '#languages' => $this->getSetting('include_locked') ? LanguageInterface::STATE_ALL : LanguageInterface::STATE_CONFIGURABLE,
    );

    return $element;
  }

  /**
   * {@inheritdoc}
   */
  public static function defaultSettings() {
    $settings = parent::defaultSettings();
    $settings['include_locked'] = TRUE;

    return $settings;
  }

  /**
   * {@inheritdoc}
   */
  public function settingsForm(array $form, FormStateInterface $form_state) {
    $element = parent::settingsForm($form, $form_state);

    $element['include_locked'] = [
      '#type' => 'checkbox',
      '#title' => $this->t('Include locked languages such as <em>Not specified</em> and <em>Not applicable</em>'),
      '#default_value' => $this->getSetting('include_locked'),
    ];

    return $element;
  }

}