I'm guessing here as I don't have access to your file, but the odds are that the two tracks are in different formats - and VLC just isn't transcoding the german audio track properly.
That can happen even in cases where VLC can play the track (for reasons I don't really understand)
You can inspect the tracks by opening in VLC, then opening menu/window/media information
that'll show you the encoding for each track for your interest.
sorry I don't have a solution