Some errors in codeigniter form_validation are not translated how to fix?

Question

CodeIgniter

In the registration form I process the parameters, the first required parameter is translated, but min_length[8] is not, I checked in all form_validation only the first parameter is translated, the subsequent ones are in English. What's my mistake? Where to dig?

$this->form_validation->set_rules(
        'new-password',
        'password',
        'trim|required|min_length[8]',
        array(
          'required' => lang('error_empty_reg_password'),
          'min_length[8]' => lang('error_reg_min_length_password')
        )
      );