This is something that really only seems like a mystery when viewed from an explicitly academic context. Speaking many languages is the norm for many people, and speaking many, many, languages is a very refined ability. When you think of language as "purely" mental and private then it seems "mysterious", but once you rid yourself of the mentalist (Cartesian in philosophical terms) confusion and focus on language as public behavior, then it's not mysterious at all.
I don't see how thinking of language as public behaviour explains anything about how some people are able to learn many languages very easily and some struggle to learn an additional one.
I don't think anyone is able to learn many languages very easily. If you want to have meaningful discussions in a language you need a vocabulary on the order of 5-10k words depending on the subject and how much exposure you have, which is a cool year of study for even the fastest learners and more akin to 2-3 for us mere mortals.
Most cases of people speaking 10+ languages lean heavily on speaking many closely related languages in a family, which most people already cope with pretty well.
Most people that know many languages, know many languages because they're trying to do things with many languages, and they grow up in that situation: they have to speak many languages. You need to get things done, so you communicate in the way that allows you to do that. They're not thinking about grammatical rules, in fact many people that learn many languages almost never learn grammar very well at all, what they learn is how to ask for a bathroom, where an address is, what's such and such called, or how do you say..., or how are you today?, how much is this?, that is okay, and various responses, ect., things that allow them to do things with people who speak different languages. Often the languages are from similar families and the peoples are from similar regions. I've met many people from the indian sub continent, for example, who can speak 4+ languages, or people from the European alps who can speak 3 or 4, quite well (which does not mean grammatically accurate in a school sense). The people that learn wildly different languages are devoting extensive amounts of time to it and are outliers. They obscure the reality that speaking many languages is a part of different communities interacting and trying to get stuff done together. It's not a private mental exercise, it's a part of public life for many.
>They're not thinking about grammatical rules, in fact many people that learn many languages almost never learn grammar very well at all, what they learn is how to ask for a bathroom, where an address is, what's such and such called, or how do you say..., or how are you today?, how much is this?, that is okay, and various responses, ect., things that allow them to do things with people who speak different languages.
Learning phrases is not learning a language. Again, you're doing nothing to explain the very real phenomenon of individual differences in second language acquisition.
My mother tongue is English, but I learned French and German for my work. My French colleagues say I "speak French" and my German colleagues say I "kinda speak German."
Would you say they're wrong? When, according to you, has someone learned a language? You don't get a medal at some point, you learn a language when you can do things with it.
