The language of the wyUpdate.exe file is english, and no matter what I do, I can't change it. It stays english.
You need to check the language boxes next to the languages you want. Then, rebuild the client. The languages are loaded based on the language of Windows. That is, if you're running the German version of Windows, the German language file will load.
When I build wyUpdate everything is ok, but when I try to build Updates I get this error message ...
That's a bug. A quick fix is to go to "File > Properties > Update & server files" and add %versionfrom% to the update filename.
Sorry about that though. I'll upload a fixed version later today.