I'm actually amazed that you found an explanation that at least makes a modicum of sense - though I don't believe you came up with it beforehand...Actually, I think most Scots' native language is Scots, which is mutually intelligible with English, but still considered a separate language.
Also if it's Scots, then it is irrelevant to translation issues, because whether or not it is considered a seperate language linguistically it is so close to English that it simply does not matter.
Again: If your world differs from the real world in any significant way, you have to tell your readers - preferably IN the story and not in a comment after you've been called out on it.Even so, his world is slightly different from ours. Differences will become more apparent as the story progresses, but that's one of them.
If you drop subtle hints in the story you will be praised for your foreshadowing when the reveal comes later. If you do it this way it just sounds like a cheap excuse - especially if there seems to be no story reason for such a difference but to excuse some other discrepancy that came up.
Oh, and if in your world Japan happens to be publicly ruled by the aliens that came to earth three years earlier, you might want to address that before it becomes an issue.