Here ( http://knoppix.net/wiki/Knoppix_Remastering_Howto ) you can find instructions about the customization of the knoppix (>7.0) distribution.
As you need to apply only "minor" changes (adding the "LANG=.." parameter as a default boot-option of the knoppix boot-loader [eg: "LANG=it" for Italian]), you need only to apply "Changes outside from chroot enviroment", as described in the related paragraph of the above linked page.
It might be useful to check other boot-related parameters that can be customized, here: http://knoppix.net/wiki/Cheat_Codes