I tend to think that the most important sign of whether somebody knows another language is more likely to be how long you would have to travel to get somewhere

I tend to think that the most important sign of whether somebody knows another language is more likely to be how long you would have to travel to get somewhere with a different native language. I don't know about the US being /uniquely/ mono-lingual, but depending on exactly where you live, you may have to drive for days or take a relatively long and expensive flight to get to the nearest country with a different language - Mexico. And so you naturally find that relatively few Americans speak other languages, and Spanish is the most common. It might be more accurate to say that Europe is unique in being highly poly-lingual, which I think is mostly due to so many countries with different languages in close proximity, all with easy border crossings and good transportation infrastructure. Many Europeans could probably get somewhere that speaks a different language in hours, by car, train, or plane. I don't think that it makes them particularly special, though.