class NegotiationSessionForm

class NegotiationSessionForm extends ConfigFormBase {
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'language_negotiation_configure_session_form';
    }
    
    /**
     * {@inheritdoc}
     */
    protected function getEditableConfigNames() {
        return [
            'language.negotiation',
        ];
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        $config = $this->config('language.negotiation');
        $form['language_negotiation_session_param'] = [
            '#title' => $this->t('Request/session parameter'),
            '#type' => 'textfield',
            '#default_value' => $config->get('session.parameter'),
            '#description' => $this->t('Name of the request/session parameter used to determine the desired language.'),
        ];
        $form_state->setRedirect('language.negotiation');
        return parent::buildForm($form, $form_state);
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        $this->config('language.negotiation')
            ->set('session.parameter', $form_state->getValue('language_negotiation_session_param'))
            ->save();
        parent::submitForm($form, $form_state);
    }

}