application/language/en/en_lang.php :
$lang['name'] = "Name";
$lang['form_required'] = "is required.";
application/language/es/es_lang.php :
$lang['name'] = "Nombre";
$lang['form_required'] = "es requiero.";
application/controllers/yourController.php :
$this->form_validation->set_rules('name', $this->lang->line('name'), 'required|alpha|xss_clean');
$this->form_validation->set_message('required', '%s ' . $this->lang->line('form_required'));
, !