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 isn't their mother tongue? What role does culture play in learning languages?