Actually, you are correct. There are lots of multilingual Americans. However, most of them are either immigrants or the children of immigrants. Among the children of non-immigrants the number of people who speak more than a few words of anything but English is pretty small.

When I originally heard this joke, the punchline had a different nationality, but I changed it, so as to poke fun at myself instead of others. What was the original nationality, you ask? Hint...I heard the joke on the BBC...