Yeah, it's a bit messy. I don't want to complicate things too much: but another detail is that your Pokemon's tag is actually determined by the language you chose for your game. Your country of origin plays no role in things whatsoever, except that you'd probably choose English if you were American, etc.
I could buy a copy of US/UM and play it in Japanese, and then get Masuda method working using Pokemon from my English S/M save file. Too bad I only know English lol.