I have found a way to change the language of the iPAQ without hex editing, checksum calculating and language code searching hassle.
I'll give an example here, in my case from German to English:
1:
Download the ROM Update of your original language, e.g. German, and the ROM Update of you target language, e.g. English.
2:
Start the target language ROM Update (e.g. English) until it says that the language of the device is wrong and break the installation with cancel.
3:
Start the original language ROM Update (e.g. German) until it successfully checks the original rom version. BUT DON'T PRESS THE START UPDATE BUTTON!
4:
Go to the temp installation folder of the original language ROM Update (e.g. c:\iPAQ\SP29835 for German) an rename the *.nbf file to *_bak.nbf.
5:
Go to the temp installation folder of the target language ROM Update (e.g. c:\iPAQ\SP29841 for English) and copy the ROM file (*.nbf) to the German folder. Rename it to the original language's *.nbf file name. (e.g. hx4700_11009_GER_AKU26.nbf for German)
6:
Go back to the installation and press the Update button.
7:
Wait
I'm a happy user of English ROM now!