Mind you, I'm saying this as a hard skeptic only because this is a persistent but weak counterargument, and thus not very useful.

The "alien mummies" and "Buga sphere" came from South America, for instance. You can find UFO and alien contact reports from all over. Brazil has the Varginha incident, the Ariel School incident in Zimbabwe, the Voronezh UFO incident in Russia, Japan Airlines Flight 1628, to name a few examples. The UFO phenomenon isn't at all limited to the US.

Although to be fair, all of that's probably the result of American pop cultural influence.