ideas :
boot knoppix normally and run :
locale -a
or
locale -a | grep be
to find out if the locale you seek to use is installed.

if it is edit the file /etc/init.d/knoppix-autoconfig and find the section that defines the language codes. check that all the data for the locale you wish to use is correct.

[it should be something like :
be)
LANG=...
CHARSET=...
...
...
;;
and many more for other language settings]