From an interesting piece on the history of reading in the LRB (apparently reading quietly is relatively modern, historically people usually read aloud), I found the following quote:
Between 2002 and 2018, the number of GCSE foreign language exams taken in England, Northern Ireland and Wales fell by 45%
This got me curious, at what rates do children learn a second language in various countries? A report by the UK government has some fantastic stats on Anglophone countries. First, the decline referenced in the above quote is almost entirely attributable to students no longer taking French. Spanish and German have only changed slightly.
That tells us about the rate of change, but what about the base rate? According to a report from the Higher Education Policy Institute, quoted in The Guardian, 32% of young people in the UK can read or write in more than one language. Even the French, notorious for refusing to speak anything but French, hover at 79%.
The 2016 census suggests that despite being far more multicultural than Europe in terms of immigrants, we lag behind in language diversity. The number of households where only English is spoken has increased. Several articles said that while 40% of students in 1960 took a second language in Year 12, it was 10% in 2016 (I could not find the provenance, so take with a grain of salt).
Data suggests that while the level is low (around 7000), the trend is stable. The figures probably underestimate the full numbers because they do not include Chinese or Arabic, which are presumably more popular than German.
About 20% of students are learning a foreign language in the US, mostly Spanish.
Eurostat has European data from 2017. As you will have noticed, this is students learning two or more foreign languages, for a total of three once you include their mother tongue. According to Pew the median rate of second language acquisition in Europe is 92%
The simplest explanation is geography. Tens of languages jostle against each other in Europe, and the common market means they have rather a lot to do with each other.
It does shed some a perspective on the assimilation versus accommodation debate going on across the Anglophone world. For all the talk about multiculturalism, there is never any question of the native population learning the language of their immigrants.