I'm guessing that in order to speak with the majority of the people on Earth easily enough that there would be no practical need for an interpreter, it would suffice to be fluent in 4 languages, in roughly this order of importance: English, Chinese, Spanish and Arabic. If they were listed by the number of native speakers, then of course Chinese would be way out in front, but of course English is the second language of a huge number of people. Each one of those 4 languages is spoken by a significant number of people other than native speakers. The number of people one could reach with English, Chinese, Spanish and Arabic might actually be much more than a simple majority of the Earth's inhabitants. It's hard to tell exactly about these sorts of things, keep in mind that I'm guessing and beware of people who claim to know the figures involved in these sorts of questions with anything close to actual accuracy.
What are the 5th- and 6th-most important languages for our hypothetical polyglot, the ones which would most effectively, in terms of sheer numbers, further shrink the parts of the human population with whom he could not speak without help? I'm guessing Swahili and French. Then maybe Russian -- lots and lots of people with Russian as a second language -- and then Portugese: less people with Portugese as a second language, but so very many native speakers there in Brazil. But maybe the speakers of Japanese, native and not, outnumber the speakers of either Russian or Portugese. I don't know. (And neither do you.)
We're up to 9 languages now. Not very many people are actually fluent in 9 or more languages, not using my standard for fluency of making an interpreter superfluous. Bear in mind, whenever people talk about how fluent they are, that there is no universally-understood standard for how fluent you have to be to legitimately call yourself fluent. My standard of not needing an interpreter is more exact than the usual standard, which is: no standard at all, but it too is a subjective call.
So what's number 10? Maybe Hindi. Or maybe Hindi is 9th, or 8th, or even 7th. Especially if one considers Hindi and Urdu to be one spoken language. That's another subjective call. There are many subjective calls when one talks about language, I can't imagine how it would be avoidable.
And then there are Indonesian and Punjabi and German and Italian. Don't ask me whether I got them in the right order, or if I've got the right top 14. For example, there's Dutch: Holland used to have vast colonies, and I don't know what sort of linguistic footprint they left behind around the world. Add that to the former huge colonies of Belgium, and the completely subjective question of whether Dutch and Flemish are 1 language or 2.
And my goodness, let's not forget Turkish. Or Amharic. But if you can become fluent in English, Chinese, Spanish, Arabic, Swahili, French, Russian, Portugese, Japanese, Hindi (including Urdu), Indonesian, Punjabi, German, Italian, Dutch (including Flemish), Turkish, Amharic -- and my goodness I completely forgot Bengali, didn't I? It should probably be in the top 10 or close to it. So if you can master those 19, or 20, or 21 languages, depending on how we're counting, plus a couple more spoken by huge populations which I undoubtedly have also forgotten -- if you're fluent in those 25 or so languages then maybe you might be justified in claiming that you, as an individual, have something like a global reach in your mind. Until then you're basically stumbling around in the dark and at the mercy of strangers like the rest of us.
This has been today's It's a big world out there kid-lecture. Thank you for your kind attention.