I think the Pokemon must be from a different language origin, so any two Pokemon coming from any English language games (this includes American, United Kingdom, and Australia) won't trigger the Masuda Method. The following are the different languages currently supported by Pokemon:
Japanese English French German Italian Spanish Korean
Yes, the two Pokemon must be from different regions, or languages in this case. The simplest method is to get a foreign Ditto, so you can breed any Pokemon. That is, of course, unless you're breeding for Egg Moves or Natures or things of that kind.
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