Form submission handler for language_negotiation_configure_url_form().
function language_negotiation_configure_url_form_submit($form, &$form_state) {
// Save selected format (prefix or domain).
config('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']);
}