I was discussing this topic in another thread and I got a lot of downvotes for suggesting that English will not forever be the world’s lingua franca. I’m not sure why people took such offence to this idea, I thought it was common knowledge that French would eventually surpass English (or even Mandarin) in terms of total users.
Anyway, I’ve linked the source of this projection. It’s a study/report from Natixis, a major corporate and investment bank (they were studying language growth to do some economic forecasting or whatever). The link to the report should be attached to this post (see page 2 for a summary, but there are subvariations of the projections and different graphs scattered all over the place in the report).
The reasoning is that most of the world is eventually going to start decreasing in population. But the world as a whole will still be growing in population. Why? Because Africa is currently experiencing a massive population boom, so the demographic weight of Africa is going to increase substantially (see, for example, the UN projections for world population growth). And of course the French language is widely spoken across Africa.
Now, is there room to critique this report? Absolutely. For instance, you could argue that it’s not fair to assume that Africa will continue to be predominately francophone; perhaps many African countries will move away from the French language now that the French colonial area is largely over. There is some movement in that direction. But regardless, this is a serious report, out of a serious institution, written by serious people. So the idea that French may surpass English a very real possibility, despite what some people seem to think.
Aviation and computers are both dramatically English. For either Mandarin or French to supplant English as the world’s most widely spoken language we would need not just a large and wealthy segment of the world that natively speaks it, but a mechanism that encourages people who know neither French nor Mandarin nor English to learn one of the former and not the latter.
French has a bunch of former colonies and a considerable bit of history where it can be a useful shared language, but I don’t know if there’s anything beyond that which would encourage someone not going to these places to learn it for casual use.
Similarly, Mandarin is the second language of a bunch of non-native speakers who live or work in China, most of which are presumably Chinese natives whose first language was a different dialect like Cantonese.
(And I believe Hindu is in a common boat, where a massive chunk if it’s second-language speakers are natives of India with a separate dialect.)
It’s likely that modern English won’t reign forever as our species common language, but I think we’re more likely to see an English-mandarin pidgin take over than we are either modern French or modern Mandarin.