[kh99]

Maybe you can try this out (it's code for a plugin using hook fetch_userinfo_query):

if (!function_exists('init_start_get_forum'))
{
function init_start_get_forum()
{
    global $vbulletin;
    if (defined('FRIENDLY_URL_LINK'))
    {
	require_once(DIR . '/includes/class_friendly_url.php');
	vB_Friendly_Url::fetchLibrary($vbulletin, FRIENDLY_URL_LINK . '|nosession');
    }

    $vbulletin->input->clean_array_gpc('r', array(
        'postid'     => TYPE_UINT,
        'threadid'   => TYPE_UINT,
        'forumid'    => TYPE_INT,
        'pollid'     => TYPE_UINT,
    ));
    $forumid = -1;
    if ($vbulletin->GPC['postid'])
    {
        $forum = $vbulletin->db->query_first("SELECT thread.forumid FROM " . TABLE_PREFIX . "post AS post
                                    LEFT JOIN " . TABLE_PREFIX . "thread AS thread ON (post.threadid = thread.threadid)
                                    WHERE post.postid = " . $vbulletin->GPC['postid']);
    }
    else if ($vbulletin->GPC['threadid'])
    {
        $forum = $vbulletin->db->query_first("SELECT forumid FROM " . TABLE_PREFIX . "thread AS thread
                                    WHERE threadid = " . $vbulletin->GPC['threadid']);
    }
    else if ($vbulletin->GPC['forumid'])
    {
        $forumid = $vbulletin->GPC['forumid'];
    }
    else if ($vbulletin->GPC['pollid'] AND THIS_SCRIPT == 'poll')
    {
        $forum = $vbulletin->db->query_first("SELECT poll.*, thread.threadid
					FROM " . TABLE_PREFIX . "poll AS poll
					INNER JOIN " . TABLE_PREFIX . "thread AS thread ON (thread.pollid = poll.pollid AND thread.open <> 10)
					WHERE poll.pollid = " . $vbulletin->GPC['pollid']);
    }
    if (is_array($forum))
    {
        $forumid = $forum['forumid'];
    }
    return $forumid;
}
}

global $bootstrap, $vbulletin;
if (get_class($bootstrap) == 'vB_Bootstrap_Forum')
{
    if (isset($vbulletin->styleid) AND !isset($vbulletin->session))
    {
        if (($fid = init_start_get_forum()) != -1)
        {
           if (in_array($fid, array(4, 5))) // force language 2 for forumids 4 and 5
            {
                $languageid = 2;
            }
        }
        unset($fid);
    }
}

Of course you must edit the code (the lines in red) to enter the forum and language ids you want.

It seems to work based on one phrase that I added to a test language, but I haven't tested it beyond that.