Do it again except download the English version.He, he...
There may be a way to reset the language, I just don't know it. It's obviously not a bios problem or the thread would have been titled, "Help! My computer speaks German and I don't!"
Go back to the Knoppix web-site, look at the FAQ and the Wiki, I'm sure there'll be an answer there for you.
http://www.knoppix.net/wiki/Language%2C_Lo...er_languages.3F