Element specific validator for the Add language button.
function language_admin_add_predefined_form_validate($form, &$form_state) {
$langcode = $form_state['values']['predefined_langcode'];
if ($langcode == 'custom') {
form_set_error('predefined_langcode', t('Fill in the language details and save the language with <em>Add custom language</em>.'));
}
else {
if ($language = language_load($langcode)) {
form_set_error('predefined_langcode', t('The language %language (%langcode) already exists.', array(
'%language' => $language->name,
'%langcode' => $langcode,
)));
}
}
}