One difference is that individuals living in multilingual communities seem to settle on an optimal cognitive load. The hyperpolyglot possesses a similar patchwork of linguistic proficiencies. Yet he or she exceeds this optimum with a conspicuous consumption of brain power (...) For multilinguals, learning languages is an act of joining society. There's no motive, no separable 'will to plasticity' that's distinct from what it means to be a part of that society. Being a hyperpolyglot means exactly the opposite. The hyperpolyglot's pursuit of many languages may be a bridge to the rest of the world, but it walls him off from his immediate language community.
Michael Erard, Babel No More: The Search for the World's Most Extraordinary Language Learners
language, polyglots