Try at next boottime which language settings you need using for example the cheatcode
Code:
knoppix lang=de
or
knoppix lang=us
If you use my graphical bootmenu then edit '/opt/knx/knoppix/boot/isolinux/knoppix.menu', otherwise edit '/opt/knx/knoppix/boot/isolinux/isolinux.cfg'.

Change within all "Append" lines the entry of 'lang=..' as you need. You can also insert in all this lines 'keyboard=..' and 'xkeyboard=..'. But I think only change of 'lang=..' will do the job. Create a new ISO and try it.

Greetings Werner * http://www.wp-schulz.de/knoppix/summary.html
Own Rescue-CD with Knoppix (Knoppix V6.7.0 remaster)