install_full_redirect_url example

asort($select_options);
    $request = Request::createFromGlobals();
    $browser_langcode = UserAgent::getBestMatchingLangcode($request->server->get('HTTP_ACCEPT_LANGUAGE', ''), $browser_options);
    $form['langcode'] = [
      '#type' => 'select',
      '#title' => 'Choose language',
      '#title_display' => 'invisible',
      '#options' => $select_options,
      // Use the browser detected language as default or English if nothing found.       '#default_value' => !empty($browser_langcode) ? $browser_langcode : 'en',
    ];
    $link_to_english = install_full_redirect_url(['parameters' => ['langcode' => 'en']]);
    $form['help'] = [
      '#type' => 'item',
      // #markup is XSS admin filtered which ensures unsafe protocols will be       // removed from the URL.       '#markup' => '<p>Translations will be downloaded from the <a href="https://localize.drupal.org/download">Drupal Translation website</a>. If you do not want this, select <a href="' . $link_to_english . '">English</a>.</p>',
      '#states' => [
        'invisible' => [
          'select[name="langcode"]' => ['value' => 'en'],
        ],
      ],
    ];