First, apt-get localepurge & remove support for all unwanted languages. Then, edit /etc/init.d/knoppix-autoconfig to set the default language to english. I think if you have done a Debian type hd install it is knoppix-hdautoconfig instead of knoppix-autoconfig; check with /etc/rcs.d/ ; the startup & config files which are used are listed there. Finally, you can remove the alternate language support files from kde with apt-get remove kde-i18n-ja kde-i18n-de etc. apt-get remove kde-i18n* will remove them all except english.
This may still not work completely; Knoppix uses special de-en versions of openoffice & mozilla which occasionaly revert to German when you least expect it. These have to be apt-get removed & replaced with standard versions. Don't know if this is true of Japanese versions though.