The general principle of this question is "If you could only speak n languages, which languages would those be to maximize the number of people that you could communicate with?" (not necessarily in their native language, just be able to communicate at all).
For example, if n = 1, then the best language would be whatever language was spoken by the most people: Mandarin Chinese.
If n = 2, it gets a little more interesting because now you need to account for the overlap between speakers of different languages. The 2nd most spoken language is Spanish but there are some people in the world who speak both Chinese and Spanish and so learning Spanish wouldn't allow help you to speak with them (because you already speak Mandarin). My guess is that the number of Spanish-Chinese speakers is relatively low and so for n = 2 your best bet is going to be Chinese and Spanish (my guess).
You get the idea by now, if I was going to speak 3 languages, it probably still follows the most spoken language trend, but at some point it will diverge. Is anyone aware of a similar list of languages that takes into account the number of people that knowing that language allows you to speak to and not just the number of people that speak that language?