How to add the Luxembourg Prov. to the subcategory section on the BE user page?
I tried a bit on the BE overall page (http://wiki.openstreetmap.org/wiki/Belgium) and activated Luxemburg (Prov.), too but will need some additional time to get it nice and link the main cities.
After adding [[Category:Users in Luxemburg (Belgium)]] to your userpage, click on the new read link.
You can create the Category:Users in Luxembourg (Belgium) and add the text [[Category:Users in Belgium]] to it.
It will make it a subcategory of Category:Users in Belgium.