My impression at this point is that AutoIt3 does unicode, but cmd.exe does not. However I can't really go to great lengths to test it. I'm not going to install a different version of Windows, for example. But if someone with a non English version of Windows wants to mess around and see if we can find out the fix, that's fine. As it is now I've added a link to the unicode info page I found on the web how to set cmd.exe prompt, to the Readme.
I have a shortcut on my desktop that I set up following those instructions but I don't think it will tell me anything since I'm on an English version of Windows anyway. Someone else needs to run the trials.
edit: The basic theme of my site since it went all donationware is it's software I wrote for my own use, that others may use at their own risk. (I think in all or most of the Readme files I have a note to that effect.) Obviously I don't have a testing department. If there's a bug or some other issue I think I can fix with a reasonable effort then I'll try to find a work-around or solution. But there's no promise of support.