backoffice/application/controllers/LanguageSwitcher.php

<?php if ( ! defined('BASEPATH')) exit('No direct script access allowed');
class LanguageSwitcher extends CI_Controller
{
    public function __construct() {
        parent::__construct();
    }

   public function switchLang($language = "") {

        $language = ($language != "") ? $language : "french";
        $this->session->set_userdata('site_lang', $language);

        redirect($_SERVER['HTTP_REFERER']);

    }
}