I think this partly depends on where you're living. Not living in China, Malaysia, or Taiwan and have no real interest in doing so? Then despite its billions of speakers, Mandarin is not all that useful. For me, an American interested in East Asia, the top five would be 1. English 2. Japanese 3. Mandarin 4. Korean 5. Spanish For someone interested in sheer numbers of people that they could communicate with, and for travelling ability, 1. English 2. Mandarin 3. French 4. Standard Arabic 5. Spanish