尝试一下：

public function index()
{
    $this->load->library('user_agent');
    $this->load->helper('url');
    if ($this->input->get("lang") =="en"){
        if ($this->agent->is_mobile()) {
          $this->load->view('menglish');
        } else {
          $this->load->view('en_signup');
        }
    } else {
        if ($this->agent->is_mobile()) {
          $this->load->view('marabic');
        } else {
          $this->load->view('ar_signup');
        }
  }
}