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Only mentioned in passing. At the beginning of Goblet of Fire, when Harry and his friends are attending the Quidditch World Cup, they pass by a couple of tents with a sign saying "The Salem Witches' Institute" - referring to Salem, Massachusetts, USA.

Also, in the Goblet of Fire movie, there's an American flag in the background waving in the breeze, just as they go into the Weasleys' tent.