I would suspect that the number of North Americans who speak a second language is comparable to the number of Europeans who speak non-European languages.
I believe the grandparent meant languages such as Japanese, Ethiopian, Thai, etc.
Uh. Yes it does, and no they aren't.
What? Nobody was saying anything of the sort.
Brazil has a large population of ethnic Japanese. A number of them are able to speak Japanese. That doesn't make Japanese a Brazilian language. Nor does it make those born and raised in Brazil Japanese, aside from discussions of ethnicity.
Finland has had a large (currently ~5%) Swedish-speaking minority for a long time. While a large amount (~50%) of Finnish-mother-tongue Finns can speak Swedish, this is expected given the Swedish minority, the previously large and prestigious role of Swedish in Finland, the relative populations (~5M vs ~9M for Sweden), and the relative linguistic isolation of Finnish.
I wouldn't claim the proximity theory holds no water - it's just not the entire story.
They know they speak español, not mexicano, they know it came from Spain, and even a Mexican will brand you as a fool if you try to tell them otherwise.
Classification of language has nothing to do with ethnicity, background, where you were born or where it is being spoken.
1. The distance. Most of the US is actually quite far away from Mexico. As you get closer to the border (Texas, California, Arizona, New Mexico) the rate at which non-hispanics speak Spanish rises considerably.
2. The economic and social relations between the US and Mexico are nothing like those between Spain and Germany. If I'm German, I can learn Spanish, move there, find work (okay, the example works better the other way around), and live with a minimum of hassle. It's another stable, developed country.
In contrast, parts of Mexico are almost literally war-zones (Juárez et. al.), and gaining a work visa, finding employment, etc is quite difficult, and even then it will be a dramatically different standard of living.