I try to change the language file for my form_validation_lang in my LanguageLoader.php in my hooks folder like this:
class LanguageLoader { function init
Error messages for form_validation are always loaded in language you set in your config.php. To change the language for your form_validation messages, you need to do a little trick, changing the default language like this:
$ci->config->set_item('language', $siteLang);