As this display needs a "base category", which I have to enter manually, this currently only works for commons, de and en wikipedia.
the toolserver.wiki table has a root_category field for this purpose - but my maintenance script for that seems to be broken, as it's NULL for almost all wikis currently :)
Anyway, the way I populate that is simple: look at the root category on the english wikipedia ("Categories"), and perhaps also on commons ("CommonsRoot"), and collect all inter-language links. That gives you root categories for all the more popular wikipedias.
HTH Daniel