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.
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.
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.
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.