The answers to this seem to ignore the fact that there are several reasons for using a language. What is important varies with purpose.
Day to day life. Includes buying things from wherever. Here the answer is 1. The native tongue(s) where you are living. 2. English There is no 3 at the moment though Chinese is elbowing its way in since if you want to buy anything that is the place to go.
Background technical information. Latin has dropped off this list for engineers and the order of the top three is subject to debate but 1. English 2. German 3. French 4. Russian 5. Japanese. Again, Chinese either should be on this list or will be soon. Detail matters here too. For example if you want computer architecture, logic, programming theory and so on English dominates. If you want closed form math, approximation methods and other things like that Russian rules.
Current hacks and technical activity. My list here is biased by my own language limitations. I won't try to order them but I have found good stuff in English, German, French and Russian (in that order). There seems to be good stuff in Chinese, but Google translate just isn't good enough to get those kinds of web information out for me.