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 English to Chinese:
1: Download the ROM Update of your original language, e.g. English, and the ROM Update of you target language, e.g. Chinese.
2: Start the target language ROM Update (e.g. Chinese) 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. English) 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 English) 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 Chinese) and copy the ROM file (*.nbf) to the English folder. Rename it to the original language's *.nbf file name. (e.g. hx4700_11009_GER_AKU26.nbf for English)
6: Go back to the installation and press the Update button.