Someone earlier said something along the lines of "you should learn Chinese because half the world speaks it!". I would like to point out the weaknesses in that argument.
First of all, learning Chinese (or any other heavily used language) is useless if you don't plan to ever live in or go to a Chinese speaking country. Let's say I plan to go live in Africa because I'm a totally selfless person and I joined the PeaceCorps. Investing the time to learn Swahili (which a very minor percentage of the world speaks) compared with a "standard" language would be much more advantageous to me. In summary, you need to consider the specific demographics of your environment(s), not the demographics of the world as a whole.
