I'm talking about which language is the most popular second language used in international dialogue, especially research.

And the relevant speed there is not population growth but how fast the interconnections and traditions of science change. On the one hand, you have a lot of new scientists coming online, but on the other hand, the bigger the network of scientists, the harder it is to change -- e.g. the lockin effects are bigger.

Rather, I think today a lingua franca is much more sticky than it was in the past, when you were talking about scientists from just a handful of nations communicating with each other. It was mostly just Europe and North America. Now, it's global. The best chance of another language displacing english isn't today, it was in the past. Now, even if China, India, and Japan all ban English and adopt, say, Japanese as their Lingua France, the rest of the world will continue with English and they'll be pulled back into the orbit. Also, there is much more trade today than in the past, so you have a network that is bigger and more interconnected, that has settled on a lingua franca, and that system has much more inertia.

It's hard to imagine what could shift this trend, especially to Mandarin. China's population is probably not going to get dramatically larger than the current 1.4 bn, which is only 20% of the world total. Maybe some sort of civilization-destroying world cataclysm like a meteor impact, pandemic, or nuclear warfare.

If I could live long enough to see the payout, I'd be willing to bet that far future humans will speak only one language, and that language will have evolved primarily from English.

But even if that language is almost certain to be English, because english changes and has so many loan words, it would probably be unrecognizable to the english we speak today.