What I am doing wrong here ? Not working.

So I started my header.php with:

$lang = $_GET['lang'];
if ($lang == 'FR' || $lang == 'EN') {
	simpleSessionSet('lang', $lang);
}

and below, I added:

<ul>
                	<li><a href="<?php echo home_url(); ?>?lang=FR">FR</a></li>
                    <li><a href="<?php echo home_url(); ?>?lang=EN">EN</a></li>
                </ul>

$siteLang = simpleSessionGet('lang', 'FR');
echo 'Session lang = '. $siteLang;

Now irrespective of whatever link I click, my $siteLang is getting echoed as FR.

Any idea ?

http://wordpress.org/extend/plugins/simple-session-support/