I speak Slovene and English fluently. I also speak Serbian, Croatian, Bosnian and Montenegrian semi-fluently (don't be impressed; since they are all based on the same dialect, knowing one language means you preety much know at least 90% of the other three languages).
Frankly, unless the one language you know is very widespread, it is quite hard to get by in today's world without knowing more than one language. Nowadays, knowing at least one foreign language is practically mandatory, with two or more being highly recommended.