From what I can find, this is no cakewalk...

Try reading this:

Subtitle & Logo Removal Guide

If this isn't disheartening enough, I found another program out there that requires you to type in all of the subtitles as you see them on the screen! (Probably not too easy if you don't speak German, eh?)

All in all, I would not recommend trying this unless you're really, really, really bored. Converting movies can be fun and challenging, but backups of movies turn out so much better! Besides, who knows when you'll next have a German visitor to entertain?

Good Luck