I found out, that the language is implemented in a simple reg key and a lot of *.mui files.
We have two different reg entries:
0409 US English
0809 ??? (probably Chinese)
In our first windows register are several files called *.dll.0409.mui, *.dll.0809.mui and lang0409.dll.
So I think, that these files are responsible for the main language of the handy. A simple test was the evidence.
I made a new entry in my registry:
After a reboot the language German appeared in the regional settings. I chose it and rebooted the handy again. The result was, that I couldn't write any text in messages, that the nice voice of speech recognation missed.
Perhaps we can now try to implement new languages in our Handy. My problem was and still is, that there is no German firmware to download. Changing some files and editing the registry is the only way to succeed! Perhaps you can give it a try!