Probably mostly true, the joke implies that it’s a bad thing to know only one language. More specifically, it’s poking fun at the fact that most Americans only speak English. But is it such a bad thing to only know English? According to this Wikipedia list of most commonly spoken languages, the most spoken “primary” language (“mother tongue” or “L1 Rank”) is, predictably, Mandarin, followed by Spanish, and then English. So, you’re at least in the top 3 just out of the gate!
But just looking at the number of speakers of their Mother Tongue (L1) is misleading, because when someone does learn a second language, it is almost always English! So instead of looking at the most common “first languages” (L1), you look at the most common languages including second languages , that is, the languages by total number of speakers (L1+L2+…), you get a very different, and somewhat surprising, list (the rightmost column in the WP table, and the main sorting rank of the table). By this measure, English is the most commonly spoken language, with 1.12 Billion speakers (at this writing), followed closely by Mandarin (1.11B), and then by Hindi, Spanish, Arabic, and French. The reason that most people learn English as a second language, if it isn’t their Mother Tongue, is that English is the de facto language of Business, Engineering, Science, and, most influentially, The Internet. Indeed, by the time you get to French, there are fewer speakers of French word-wide than the population of the US! So, really, there are only five global languages: English, Mandarin, Hindi, Spanish, and Arabic. So if you were going to learn a second natural language, you’d do best to learn one of those.