Mezzofanti's Gift

Multilingualism is on the rise in the coming decades, as many as two billion people will learn English as a second language. The next stage up from multilingualism is the domain of the hyperpolyglot or superlearner: someone who claims to know at least six languages. But what does it mean to know a language? Can a person claim to speak a language fluently if it isnt their mother tongue? What role does culture play in learning languages?