Implements \Drupal\Core\Form\FormInterface::submitForm().
Overrides SystemConfigFormBase::submitForm
public function submitForm(array &$form, array &$form_state) {
// Save selected format (prefix or domain).
$this->configFactory
->get('language.negotiation')
->set('url.source', $form_state['values']['language_negotiation_url_part'])
->save();
// Save new domain and prefix values.
language_negotiation_url_prefixes_save($form_state['values']['prefix']);
language_negotiation_url_domains_save($form_state['values']['domain']);
parent::submitForm($form, $form_state);
}