I don't remember saying that UTF-8 was a locale. What I said was that a locale can use UTF-8 encoded text instead of a 2-byte encoding such as ISO-2022-JP. I don't remember saying it should be treated as ASCII either.
I have noted, though, that Windows uses UTF-16 (Or UCS-2 as you call it) internally. UTF-16 is a transition format just as UTF-8, and isn't significantly different.
