I don't know your reality but literally anywhere in the 744 million users in Europe if you consider the technology literate average of internet users I guarantee that someone who is not even bilingual is precisely the exception.
I would hazard say the same is true in most of Asia and Africa perhaps less so in South America where Spanish/Portuguese are more monolithic.
I don't think you understand the proficiency needed to comfortably consume content in another language. Save for a few hotspots (Benelux, Nordics) most people say "yes, I speak a second language" because when they focus hard, they can say "me wants toilet" and that's enough for their use case (holiday abroad once a year). My mother is a teacher of English but when I brought a friend who only spoke English, I had to translate the conversation between them. This is the reality of bilingualism in small towns.
I don't agree with the average user not being bilingual, but I do see your point about who the target audience is here.
I don't have any numbers but it seems plausible that the average user doesn't make an informed decision about where they consume their videos or which search engine they use. They buy a smartphone to connect to their family and friends, to learn and to use essential services.
Unfortunately the smartphone is not designed to help users with this goal. It is a medium for tech companies to shove garbage down the throats on people who are just trying to live their lives, and to perform the largest mass surveillance campaign in the history of mankind. Communication and connectivity is the Trojan horse used to sell the malware that the smartphone is.
This feature is designed for users who "don't know any better".
As others have commented already: Most people in Europe can speak at least two languages, and even the minority who can just speak one can typically understand several. Even my old parents, when they were alive, could understand at least three languages, and English wasn't one of them. Africa? Most people can handle several languages. South Africa, for example, has 11 official languages, people can often speak a bunch of them and understand several more.
Asia? Varies, but multi-lingual is common in many regions, though not so much in Japan.
It's not that you have to be able to speak multiple languages, understanding several is still common.
I think it's more that the YouTube developers are US Americans who overwhelmingly speak only one language and naturally assume this is the case everywhere else.
This is a clear example where diversity of lived experiences make for a better product. I have no doubt that a young monolingual US developer can't imagine that many, if not most, people around the world are juggling between two or more languages multiple times a day. But it's obvious to anyone else.
This feels off to me. There are so many Indians in silicon Valley, most of which speak at least one Indian language (even if not as well as English) in addition to English, some from India, some whose parents or grandparents are from India. They're both there in higher levels and lower levels - they're throughout the company, and none of them stepped in? I don't think this makes sense.
Most sources I can find claim that around 60% of the world's population is multilingual. I suspect that the number of internet users that are multilingual is probably pretty close to that.
