According to decree 78/1991, section 3.3, all place names in guides and descriptions of Catalunya for use in Catalunya must use the official Catalan spelling. No more Catalonia, Cataluña, or Catalogne. Katalonien is most definitely verboten. (and this guy is definitely going to jail)
There's an exception if you live in the Aran Valley, they are allowed to call it Catalonha.
Does a blog from Sant Cugat for Santcugatencs qualify? It's not clear (there is an exception for textbooks used in schools that are written in a foreign language), but to be on the safe side, I will try to use the official spelling.
So there, Tom. :)