French and Arabic are the second and third most spoken language in number of countries. Then there is the obvious Mandarin which is spoken in most of China with around a billion locutors
I keep debating Mandarin but my issue is how the language is tied tightly to China. Helpful if I decide to explore China in depth but seemingly less so if I want to "get by" in a large number of countries. If I had an ability to learn languages quickly, I would probably learn French, Mandarin, Arabic, and Hindi, but I think I am already pushing my limits.
Yeah, that's the thing: "which language is spoken by the most people" is an easy question to answer, but "which language (or combination of languages) lets me communicate well enough to get by in the most places" is much harder because the statistics aren't necessarily collected in a way that lend themselves to that kind of analysis.
For example, Hindi is spoken by a whole bunch of people, but I'm pretty sure the vast majority of those people also speak English, so if you already know English you don't actually need to learn it.