I might be wrong with this but it may have something to do with your Gamer tag. I've heard people not being able to get DLC in there language or content that should be available to them but isn't usually because they made there Gamer tag in one country and moved to another. You say you live in Austria? did you move to Austria with your console and Gamer tag? If that is the case you would have to contact Microsoft to help and if it isn't it looks like you would still have to call them about the DLC and see if It's in German or not.
In my setup it says I live in Austria. This does effect some things, I can't download Indie titles for one thing. Usually for multilingual titles there's a download and either all languages are in there or at some point the system figures out what I need (I'm not entirely sure how it sorts this out, but it works). I bought all the Dragon Age DLC, for example, and didn't have a problem (although sometimes the description of the DLC was in German, the DLC played in English).
There are a couple of issues with living in the "wrong" country for Xbox Live, but mostly they are cosmetic. I had to buy the DLC for Fallout 3 from Amazon when it came on disc, but I would have bought all of it online given the choice (even though it costs more).