You need to ensure that your language is included in /etc/init.d/knoppix-autoconfig. An example for German (de):
de)
# German version
COUNTRY="de"
LANG="de_DE@euro"
KEYTABLE="de-latin1-nodeadkeys"
XKEYBOARD="de"
KDEKEYBOARD="de"
CHARSET="iso8859-15"
# Additional KDE Keyboards
KDEKEYBOARDS="us,fr"
TZ="Europe/Berlin"
;;

If it is not then add it.