This is how to modify/replace one library file to have support for diacritics for "s", "t" and "z" in XTNDConnectPC.
Process translation:
During instalation of XTNDConnectPC library "rstSiemens2.dll" was copied into "Program Files\Common Files\XCPCSync\Translators\".
You need to replace it with http://www.gis-service.cz/gsm/siemens/sme45/rstSiemens2.zip
After that you have to "activate" czech language:
1. Start Outlook
2. Backup all you contacts you have in Oulook
3. Delete addressbook in mobilephone
4. Connect your mobile to PC
5. Synchronize
This is enough for Czech language. There is no support for other special characters (e.g. Slovak).
Procedure is one year old, so you may find some newer version somewhere.