Quartz – Emily Oster 
First, let me say I don’t think you are thinking about this quite the right way. You asked me about what language would be most useful. This suggests you’re thinking about only one side of the choice: the benefit side. Making the optimal choice requires thinking about both benefits and costs. In this case, I’d argue there may be very different costs to acquiring proficiency in different languages. It is a lot harder for most English-speakers to learn Japanese than to learn Spanish. But let’s start with the benefits. The benefits of a new language arise from the new people you can interact with in your new language. If maximizing this was your only goal, Mandarin would be the best choice: This is the native language for 14% of the world’s population, and most of those people do not speak English, so it’s all a win.(more)