Gregor,
This is probably because you have an old version of _ function. You can check it from /api-doc and then searching for "_". If the function is in lang-procs.tcl and takes both the locale and the key as arguments, it's old. There is no lang-procs.tcl file anymore. The _ proc should be in lang-message-procs.tcl and take the key as its first argument.
I ran into this same problem after updating my code from cvs. As I still had the old lang-procs.tcl and it got sourced after lang-message-procs.tcl, I got the wrong version of _ proc.
You should be fine when you remove the lang-procs.tcl and see that you have the latest version of _ proc.
