In California, my social circle is mostly PhDs. These are phenomenally smart people. You go to a JPL party and the average IQ has to be 2 or 3 standard deviations above the mean. There are a lot of rooms where I'm by far the dumbest person. But most people I know are either unilingual or bilingual. Even in Southern California, a majority of people have sub-conversational levels of Spanish. I know very few people who are conversationally trilingual. Where I grew up in Canada, E/F bilingualism was somewhat common but very, very, very few people spoke a third language and most people had halting French. I'm in Ireland right now and native-born Irish speak English and a culpa focal of Irish, and that's about it.
What this says to me is that it's pretty unlikely that a country's tendency towards multilingualism is meaningfully correlated with education or economic status. And while I know the idea of measuring "intelligence" is completely fraught, I can at least say it seems unlikely that a random sample of people in one place are as intelligent as JPL people.
All this to say that in all three of the anglosphere countries I've lived in, people seem to have a mental block on the idea that a second language is essential, or that a third or fourth language are even feasible. Someone who fluently speaks four languages is considered a polyglot, a freakshow, some kind of Beautiful Mind. But then you go to Netherlands or Switzerland or Morocco and it's normal. And the reason for this isn't obvious to me: it doesn't seem to relate to income, education, it doesn't seem to obviously correlate with intelligence. It seems to just reflect some kind of cultural premium placed on language acquisition. I'd love to read more about why this is.