I can't read German ... Ok
1) Go to dir ./etc/init.d and find this files:
knoppix-autoconfig
knoppix-autoconfig.orig
2) edit both and search for this string
### localization
# Allow language specification via commandline. The default language
# will be overridden via "lang=de" boot commandline
LANGUAGE="$(getbootparam lang 2>/dev/null)"
[ -n "$LANGUAGE" ] || LANGUAGE="de"
3) now change in last line LANGUAGE="de" for what you want, maybe LANGUAGE="us",LANGUAGE="uk", etc.
I had change both files but i don't know if you need both.
Try only the first, if don't work try the other.
Bye.