While dealing with my customers language files, I came up with a nice Idea for having a language switch between the provided english language and my newly created german language (which I could provide here, when it's finished and you want me to):

Look for
PHP Code:
if (! isset($_SESSION['LANG']))
 {
  require_once 
VSHARE_DIR '/include/class.language.php';
  
language::cookie();
 } 
and change it to
PHP Code:
if(!isset($_GET['lang'])){
 if (! isset(
$_SESSION['LANG']))
 {
  require_once 
VSHARE_DIR '/include/class.language.php';
  
language::cookie();
 }
} else {
 require_once 
VSHARE_DIR '/include/class.language.php';
 
language::validate($_GET['lang']);

All you need now is a link to your actual page with the addition of ?lang=nameyourfolderhere
Seems it works for me ...