There are indeed language geniuses (hyperpolyglots) who are fluent in even much more than five languages, see, e.g., this article: https://www.newyorker.com/magazine/2018/09/03/the-mystery-of-people-who-speak-dozens-of-languages. But this ability is rare.
i too would question their competence. It is possible to have a basic knowledge of several, but not usually to native speaker level. So it depends on what you mean by fluent. How fluent?
Valerie Saunders and Wolfgang R. Dick many thanks for your input. I think you are right, and there exist hyperpolyglots, but they ARE few in number. What I believe in is that language exposure is one of the most intense cognitive training regimes one can go through. As a result, multilingualism can induce profound neural changes or “neuroplasticity” (Costa and Sebastián-Gallés, 2014) Article How does the bilingual experience sculpt the brain?
. Obviously, learning a new language in adulthood can change your brain. What remains unclear is the scope of such neuroplasticity, or in other words, what are the limits to language acquisition and how you can push them?
I will ask you more questions, if you don`t mind. How many languages do you speak? And how do you “encourage” your brain to make further progress?
I speak French and Japanese. I do try to learn bits of other languages such as Italian and German. I also encourage my brain to make more effort to develop cognitive skills.
Valerie Saunders My idea is that it is easier to master the second or the third language if they are typologically distant, like French and Japanese. While learning genetically related languages makes your brain work way harder. For example, people proficient in English tend to face certain difficulties while learning German (I judge from my own experience and from my students` feedback). Do you agree?