OK, then I don't see your problem - sounds like you already know what you need to do and just need to create the specific tests to run against the application - prerequisistes being that multiple iterations are performed on a non-english language system.
If you're testing asian languages then you may see conflicts if the app is designed only for ASCII use because some languages are Unicode, not ASCII. I don't know if this will cause a problem at the operating system level or not. I've never run Chinese Windows. I've added language packs, but not the operating system specifically released in that country. I don't know if there would be any difference even.
I don`t see the way for automated testing look-and-feel of your site under the different languages of the OS.
But I think, that it`s not a huge problem, because this functional tests won`t be run very frequently.