NegotiationSelectedForm.php

<?php

namespace Drupal\language\Form;

use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Language\LanguageInterface;
use Drupal\Core\Form\ConfigFormBase;

/**
 * Configure the selected language negotiation method for this site.
 *
 * @internal
 */
class NegotiationSelectedForm extends ConfigFormBase {

  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'language_negotiation_configure_selected_form';
  }

  /**
   * {@inheritdoc}
   */
  protected function getEditableConfigNames() {
    return [
      'language.negotiation',
    ];
  }

  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {
    $config = $this
      ->config('language.negotiation');
    $form['selected_langcode'] = [
      '#type' => 'language_select',
      '#title' => $this
        ->t('Language'),
      '#languages' => LanguageInterface::STATE_CONFIGURABLE | LanguageInterface::STATE_SITE_DEFAULT,
      '#default_value' => $config
        ->get('selected_langcode'),
    ];
    return parent::buildForm($form, $form_state);
  }

  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    $this
      ->config('language.negotiation')
      ->set('selected_langcode', $form_state
      ->getValue('selected_langcode'))
      ->save();
    parent::submitForm($form, $form_state);
  }

}