While learning all those languages is certainly no easy task, I'd think keeping them all straight afterward would be a pretty impossible feat in itself. He could know fifty words for hello but probably forget which one is correct for the language he's speaking at the moment. At least, I would.
I learned this one just this week when the Google blog had an article about a guy who was one and speaks over 50-something languages - wow.
A hyperpolyglot is one who can speak six or more languages fluently. The term was coined by the linguist Richard Hudson in 2003 and derives from the word "polyglot", meaning one who can speak multiple languages. Hyperpolyglot on Wikipedia