Change the language dynamically?

I am attempting to change the system language dynamically with a call to a controller method. I have in the view :

PHP Code:

<?php if ($this->config->item('language') == "francais") : ?>
<a href="changelangue?lang=english" data-js-langue>EN</a> 
<?php elseif ($this->config->item('language') == "english"):?>
<a href="changelangue?lang=francais" data-js-langue>FR</a>
<?php endif;?>

And in the controller I have this method:

PHP Code:

public function changeLangue(){

    if (!isset($_GET['lang']))
    {
        redirect("vehicules");
    }

    $this->config->set_item('language', $this->input->get('lang'));
    redirect("vehicules");
} 


I am being redirected, but the config language is not changing. It seems to always revert to the default language. I even tried changing it in the ' __construct' of the controller in question using the session userdata like so:

PHP Code:

public function __construct()
{
    parent::__construct();
    if (!$this->session->userdata('language'))
    {
        $this->session->set_userdata('language', $this->config->item('language'));
    }else{
        $this->config->set_item("language", $this->session->userdata('language'));
    }
} 


I have the language folders in system/language and in language.

Am I missing something?