hippak,
Welcome to this superb website and forum.
The instruction below tell you how to backup the image of 4150, using the command "r2sd" (ROM 2 SD).
http://www.geocities.com/ipaq41xx/
But if you have the English ROM on an SD card (honestly I don't know what format that has to be in), you might be able to use "sd2r" to change the language to English, providing you edit the English ROM to fake as "French".
In order to change the ROM language, there has already been many post on this forum.
Good luck.