Hey Kai,
I believe that this is more a FaLang issue. We use getChildren to get the array with our categories. Here is the code:
$cats = JCategories::getInstance('Hotspots')->get()->getChildren(true);
This is the defualt joomla "way" to deal with categories. My guess is that FaLang has some added code to go around the langauge issues. Can you contact them and let them know of the issue. Let me know what they say. (Also give them a link to this thread. If they want they can contact me for assistance)
Regards,
Daniel