UserLanguageTestForm.php

<?php

namespace Drupal\user_language_test\Form;

use Drupal\Core\Form\FormBase;
use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Url;

/**
 * Provides a User Language Test form.
 */
class UserLanguageTestForm extends FormBase {

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

  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {
    $form['#action'] = Url::fromRoute('user_language_test.post_response')
      ->toString();
    $form['actions'] = [
      '#type' => 'actions',
      'submit' => [
        '#type' => 'submit',
        '#value' => $this
          ->t('Send'),
      ],
    ];
    return $form;
  }

  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
  }

}