class SelectLanguageForm

class SelectLanguageForm extends FormBase {
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'install_select_language_form';
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state, $install_state = NULL) {
        if (count($install_state['translations']) > 1) {
            $files = $install_state['translations'];
        }
        else {
            $files = [];
        }
        $standard_languages = LanguageManager::getStandardLanguageList();
        $select_options = [];
        $browser_options = [];
        $form['#title'] = 'Choose language';
        // Build a select list with language names in native language for the user
        // to choose from. And build a list of available languages for the browser
        // to select the language default from.
        // Select lists based on all standard languages.
        foreach ($standard_languages as $langcode => $language_names) {
            $select_options[$langcode] = $language_names[1];
            $browser_options[$langcode] = $langcode;
        }
        // Add languages based on language files in the translations directory.
        if (count($files)) {
            foreach ($files as $langcode => $uri) {
                $select_options[$langcode] = isset($standard_languages[$langcode]) ? $standard_languages[$langcode][1] : $langcode;
                $browser_options[$langcode] = $langcode;
            }
        }
        asort($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',
                    ],
                ],
            ],
        ];
        $form['actions'] = [
            '#type' => 'actions',
        ];
        $form['actions']['submit'] = [
            '#type' => 'submit',
            '#value' => 'Save and continue',
            '#button_type' => 'primary',
        ];
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        $build_info = $form_state->getBuildInfo();
        $build_info['args'][0]['parameters']['langcode'] = $form_state->getValue('langcode');
        $form_state->setBuildInfo($build_info);
    }

}