It's a translated rehash of a quote, by French writer Claude Gagnière: Un homme qui parle trois langues est trilingue. Un homme qui parle deux langues est bilingue. Un homme qui ne parle qu'une langue est anglais. In in your case, someone swapped "English" with "American". The jibe is that a native English speaker doesn't speak any other language, nor sees any need to do so.

What do you call a person who speaks no languages? A monkey.

I heard this joke recently from my English teacher and I laughed. I told the joke to Mom (who laughs at every joke I tell her) but she got upset at that joke. She said (and I guess this is my answer), "Half of all Americans do know another language. The half that are bilingual are from another country (most likely speaking Spanish). The other half who only speak English did not feel the need to learn another language. It is not wise to judge people." Then again, half the people I know speak only English.

Wow I got it. Its funny if your use to a sort of nationalistic joke. Basically its a joke on americans saying that they can't speak more than one language. I don't know if you'd find it funny but it's hilarious to most people

This joke implies that Americans are uncultured, that the majority of Americans only speak one language. This joke would make sense if you had an impression that Americans were uncultured.