class NegotiationSelectedForm

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);
    }

}