Problem fixed - the script author was kind enough to email me directly. Thanks Chris!

The line:

python knoppixlang.py --lang=us KNOPPIX_V3.4-2004-05-17-DE.iso

has to be entered in the WINDOWS command prompt, not the python shell! You then type 'y' when prompted to replace the x instances of 'de' in the iso with 'us'.

Script works very well - my German iso was completely changed to English. All menus etc in the burnt boot CD are english.

BTW: as downloaded the iso is read-only. File permissions for the iso need to be changed to allow writing by deselecting 'read-only' in the file properties (right-click on file icon) before running the script, otherwise you get "permission denied".

Strongly recommended.