All but a handful of EU countries. Lived in DK, ES and BG. I can tell you that very few teenagers speak English well enough to learn to program so they can start a Facebook at 18 year.
When I was a European teenager in the 1980s, I met many peers who were great programmers - given their age (around 15, 16). They all spoke English very badly. They just weren't interested in English, only in programming languages. These were self-taught kids who were also good in math and science. Apparently you don't need to know much English to get good at programming.
I think the argument was that a non-English Facebook likely wouldn't have taken off the way it has.