This is not quite a “which languages have the greatest number of native speakers” question, nor quite a “which languages have the greatest number of L2 speakers” question, both of which are easily googlable and come up plenty of times when I try to google this question. Also, this question does not give me the answer I’m looking for: all of its answers focus only on native speakers or ignore the fact that there is overlap in L2 languages.
Say I’m a language learner who is choosing languages to learn to maximize the number of people I can communicate with. Good bets to learn seem to be Mandarin and English, with the former’s high L1 population and the latter’s high L2 population. But if I look at Cantonese, it’s not a good choice even though it may have a large number of speakers: statistically, most people who speak Cantonese also speak Mandarin, even though their Mandarin is likely an L2 or L3. So even though we’d both use a second language to do it, I could communicaye to them in Mandarin, which has more speakers. By this criterion, it’s inefficient to learn two languages with a large overlap in speakers.
My question is this: is there an ordered list such that, starting at the top, each additional language down the list maximizes new speakers who do not already speak another language higher on the list? I’m pretty sure English tops the list, with its high L2 population, but what comes after that?