But there are probably still some words that will make me think the person is not American, like organisation with an s. I read it and it viscerally feels off.
Due to the large number of U.S. internet residents, I don't tend to really notice many "Americanisms", though one thing that bugs me is when the U.S. doesn't seem to recognise that "America" is a a large continent and only one area is the U.S. However, even Canadians will refuse to admit that they are American, as in living within the american continent.
Perhaps the solution is to just unite all of the states of America, because I at least think the original idea was that the USA was a supranational government uniting little nation-states (precursor to the EU?), but maybe these more established nation-states won't want to join the union and become a "state" (even though I think political science would already refer to them as states).