Sounds like you downloaded the version with DE in the name and no the version with EN in the name. See the downloading faq for details.

You used a mirror? Much better and faster to use Bittorrent. See the downloading faq for details.

You can run your copy in English if you want. See the cheat codes faq for details.

Of all the places where you could have posted this, it doesn't relly fit in Other Live CDs, as you are talking about Knoppix, not an other live CD, it will be moved.