Welcome to Aximusers.com
First off, their is no Dutch version of WM2003.
You are I am sure aware that you can change the regional settings on your X50v so that much of the menues are in Dutch ??, True, the applications will be in German unless you have purchased 2nd party programs in English of Dutch. I know as I used a German x5 for GPS for approx 18 months before I changed to English.
Please consider if the above meets your needs before you look to change the ROM language as this action does carry a high risk.
If you still wish to change the ROM language from German to English, then take a look at the following link
rom update gone bad... any1 can save my axim?
Ignore the title ( although it is a good warning) and about 4th post in, you will find the link to the process for flashing the ROM. Make sure you have the correct file, as this person used the WM5 to WM2003SE downgrade file and effectively created a paperweight from his X50v.
I wish you luck, but again you should consider carefully the high risk in attempting this process.