Builds the selected language negotiation method configuration form.
function language_negotiation_configure_selected_form($form, &$form_state) {
$form['selected_langcode'] = array(
'#type' => 'language_select',
'#title' => t('Language'),
'#languages' => LANGUAGE_CONFIGURABLE | LANGUAGE_SITE_DEFAULT,
'#default_value' => config('language.negotiation')
->get('selected_langcode'),
);
return system_config_form($form, $form_state);
}