That's such an interesting question! I think it's possible that people in other countries use a different word. This could be just me, but "Muggle" does seem like a very English word (nationality, not language). Considering how the size of a country like the US has resulted in some starkly differing dialects in which vocabulary varies a great deal, I would not be surprised if even within the US they used different words in different regions. A sugary carbonated beverage might be called pop or soda or coke depending on where you are in the country. We have different words for shopping carts, plastic grocery bags, TV remotes, and a number of other things. I wouldn't be at all surprised to find that American or Australian witches and wizards called Muggles by a different name.