Hrm... well the indicator won't work on XP at all... actually, now that I think of it, XP uses a different mechanism and doesn't even have internat.exe. So maybe you could get it to work by using an internat.exe from Win2k... I'll have to try that.
I wonder how/where XP loads the language indicator without internat.exe...
Do you know if the "registering" is done via window messages?