I am forever amazed, and somewhat shamed, to see on the news some Afghani out in the middle of some swirling dust pit speaking better English than I speak the prevalent non-English language spoken around here. When and how did they learn?
Americans have always been low in foreign languages (though in grad school, I was told by a Japanese student that Japan had been the same). To learn a foreign language, you have to have a reason to learn it, as well as exposure. Americans can usually get along quite well without a word of another language, and even if they try to learn one, they have to go out of their way to get exposure to native speakers. I understand that in Europe, in contrast, it's not unusual to go to another country for a day's shopping or a weekend trip.
I had schoolmates in grammar school who were pretty good in their immigrant grandparents' language until the grandparents died. The parents were most uninterested in keeping it up, and the kids forgot over the next few years.
Also, English has for years been the language to learn. When I was studying Hebrew 20 years ago, I heard of the difficulties my classmates had trying to improve their Hebrew in Israel (which at that time had 8 years of required English in the schools) -- once an Israeli knew you were American, he was only interested in practicing his English.
One guy in my classes said I should go to Israel for my Hebrew. He started to say that even keeping the TV on would help -- then he recalled how much of Israeli TV was American shows with Hebrew subtitles!
We now have 5 Spanish language channels on our basic/extended cable TV. My facility with spoken Spanish is the best it's ever been now that I have "at will" access to it.