the default language setup is in a file called "knoppix-autoconfig" which I beleive is in /etc/init.d/

I can't tell you the exact settings to "tweak" to get it working perfectly and if you find the solution I'd really like to hear what you did exactly cause I'm having some problems getting it to work with a distro called kurumin which is a distro based in brazil and I can't seem to get in touch with anybody else who's sucessfully changed the default language.

I do hope this helps you.