Russia is a rising country, really? But anyway I think the Japanese example as I said above (English/Japanese bilingual signs at the Minneapolis airport looks kind of strange now) shows how you can’t guess stuff like that.
Also the morphological tonality in Chinese spoken dialects as well as huge number of symbols to learn makes it an unlikely candidate for the international language. It isn’t just a question of when X country GDP gets to be biggest. A whole lot of other things have to change, and there’s a whole lot of sunk investment of all kinds in English and learning English. Again hard to predict but I’d guess English is going to be last de facto international language and what follows will probably be seamless enough machine translation that it doesn’t matter what language you speak, for most purposes, not all purposes. Whether that’s years or several decades away I don’t know.
Today’s situation of computer assist is already a big change from what it was, and opened up lots of different useful levels of language learning. For example I know some languages (eg. Spanish, Italian, German) just well enough to find what I need on the web and translate it well enough, I also do that for web material in languages where I can read whole pages, but I’m slow and have to look a lot of words up (eg. Japanese). In Korean OTOH it’s become about as easy for me to read and look up words as figure out what a garbled machine translation means (Google does Korean worse than most languages, though noticeably better than it did only a few years ago). And if a source is non-electronic (book, document or mag article) I’ll generally just plow through it reading, slowly if Japanese or Korean, French I can actually read at good speed with fairly little dictionary reference, once I get warmed up. But I’ve done a fair amount of research in Russian by scanning, OCR’ing and machine translating non-electronic texts. That’s drudgery (because scan/OCR programs still make lots of mistakes you have to correct first), but does save the upfront investment of improving my Russian enough to just read big texts.
Such variations were impossible years ago. It was more like you either knew the language or not. So the idea of studying languages to ‘proficiency’ for all uses is itself arguably becoming obsolete. Depends what you want to do.