For some reason you can't download the FULL German version out of "Your Games"-section (yes, even if you select "German" in the pull down-menu).
The only way (so far) to get the German audio is from the frontpage, you know where all the trailers are. A pop-up should open where you can choose which version you'd like to download (mac/PC, German/English). Just download the German mac-version. If you've accidentally closed the pop-up, refresh your cache and it should appear again.
After downloading, decompress the file. You should have a file called "BackToTheFuture101 blablasomething" (forgot how it was called, sorry) now. I don't know why but to get the game started you have to rename it! Just rename it into "BackToTheFuture101" and it should work, now completely in German! (at least that's how it worked for me) :)