qTranslate slug, get url of post in other language in Ajax call

I’m making a bilingual WordPress site. I want to load posts via AJAX and have a link to the same post, but in the other language. I’m using qTranslate slug and usually I rely on the global $qtranslate_slug variable like so:

$qtranslate_slug->get_current_url($targetLang);

This works fine if you are on post page, but it doesn’t work in an ajax loaded post.

In my AJAX call, I’m setting the post object this way:

$post = get_post($postId);
setup_postdata($post);

I get all the data from the post, I’m just not sure what function to call to set the $qtranslate_slug object. In the ajax call, $qtranslate_slug->current_url is an empty array.

UPDATE 1

This is probably not the right way, but I called wp() in my ajax call and it set the $qTranslate_slug object. HOWEVER, the links are “ugly” i.e. http://example.com/fr/?id=68
How can i get the pretty url?

UPDATE 2
in functions.php, i have the following function:

function t($fr, $en){
    switch(qtrans_getLanguage()){
        case "fr":
            return $fr;
        default:
            return $en;
    }
}

Here’s my ajax code:

<?php
global $qtranslate_slug;
if(isset($_GET["id"])){
    $siteRoot = dirname(__FILE__) . "/../../../../";
    require_once $siteRoot . "wp-load.php";
    wp();
    $post = get_post($_GET["id"]);
    setup_postdata($post);

    // generate post html. I didn't include it here, because it has nothing to do with the problem.
    ?>
    <script>
        var alternateUrl = "<?php echo $qtranslate_slug->get_current_url(t('en', 'fr')); ?>";
    </script>

<?php } ?>

UPDATE 3

qTranslate gives you a url in the style of: http://example.com/fr/?id=68
If the “id” is changed to “p”, the url will be converted to a pretty url when you visit the page. However, I would still like to know how to get the pretty version to be displayed in the link.