The idea that we would all get along better, or perhaps progress scientifically if we all spoke the same language has a long steady minor popularity, never disappearing but never becoming much of a force either. I have never thought the getting-along part was particularly persuasive. Serbs and Croats speak the same language, America's bloodiest war was between sides which both spoke English, and worldwide, civil wars often include combatants who understand each other quite well, thank you - and that's the problem. Is it worse when opponents also speak different languages? Perhaps. I might even say probably. But mutual intelligibility is at best a percentage improvement, not a cure.
Does shared language improve scientific progress? It seems to have worked with Latin in Europe in the Middle Ages, and American science PhD's used to require facility in a language used in one's field, both for reading articles and interacting with peers in other countries. German and French were favored for this when I was in school, though Russian was popular in some fields.
Esperanto, that favorite of European idealists since the 19th C still has adherents. It looked like it was going to disappear after a century of mild success, but the internet has created a new community of speakers and writers. Still, the growth has not been explosive, as other communication changes related to the internet have been. While there are East Asians who learn it, it is going to attract few learners whose main language is not European and even Western European. There are some Slavic elements, but it has largely Romance and Germanic foundations. If you originally speak Japanese, your effort is going into English, not Esperanto. It has a popularity among those who resent Americans and the domination of English on the international stage. But then, you have to take an artificial stand that you, Urdu-speaker, are going to side with the rest of Western Civilisation instead? Unlikely.
Interlingua, whatever its virtues, has very few users and is very heavily Western European. There is no incentive for a speaker of Mandarin, Tamil, or any of the Arabics to learn it. Not gonna happen. There was also an invented language by a Swiss priest in the 19th C. Began with a "K," I think.* Never caught on anywhere.
So the constructed languages don't seemed to have achieved enough velocity for liftoff.
That leaves us with languages currently spoken, or simplified versions of them. French was the language of diplomacy and so known throughout the world and even favored in those places where English was hated. But French has faded. If the world were only the Western Hemisphere, some Spanish-Portuguese combo might win out over English and certainly French. But there is more to the world than that, much more. One might think that English-speakers would universally prefer that their own language remain the standard international language, but there is plenty of sentiment that it's just not fair, and symbolic of colonialism and capitalism and American hegemony and boy-we-hate-McDonalds-and-our-ugly-culture. Not that these folks are going out and learning Mandarin in any numbers, which would be the first credible alternative. They might know some nice European language to show their internationalist bona fides, but their advocacy for anything else is basically that schoolchildren should be made to learn whatever, because America is on it's way out, dammit, and we need to stick it to those Trumpsters and other jingoists. Anyway, it's the kids who will do that work, not them. Many (not all) who are anti-English because of cultural imperialism smuggle in Whorfian ideas, the belief that one's language influences or even dictates how one sees the world, also called The Language Hoax.
Because Mandarin, even though it starts with an enormous advantage of a billion speakers right out of the gate, is hard to learn. It has tones, which are very difficult to even imitate, let alone master, if your ears did not start picking them up before age five. It is not like learning bad English or bad French, where your mispronunciations can be pieced together by natives. Mandarin is based on single syllables with varying tones, and getting the tone wrong does not mean you are saying "woman" badly, but another word entirely, multiple times in each sentence. Further, speakers of Indo-European languages in general, and very especially Americans, are used to non-native speakers butchering their language yet understanding them anyway.**
More importantly, whether it's fair or not, English got in first, learning the languages of other people at least in part in order to trade, and teaching its own tongue at the same time. English is not just the language of airlines, but of travel in general, and most places like tourists even when they hate them. American culture has permeated the world. K-Pop, Bollywood, and other Asian popular expressions are on the rise, sure, but they are starting from so far back as to be nearly invisible. China will gain traction because of numbers alone. They apparently already quietly influence Hollywood's choice in ways we seldom notice.
There are simplified forms of English used internationally, such as ELF, Basic English, and Global English or Globish, which native speakers are not that familiar with. While supposedly aiming for a standard small-e english that can be understood by Magyars and Malagasys who have no other language in common, in practice each region develops its own versions. Not a terrible thing, I understand, as there is still a lot of intelligibility and it allows people to feel some control.
But doesn't English spelling ruin everything so much that it just has to fall into disfavor eventually? I recall as far back as elementary school reading SRA articles moaning about enough, cough, through, though, bough, thought. (Crazy spelling happened because literacy was becoming more widespread even before the printing press, and just as there were more people who could write things down in English, the Great Vowel Shift of the 15th-16th C occurred, rendering a lot of the spellings obsolete. Spellings change much more slowly than spoken language, and so are retained.) Yes, that's a problem. But it's not a problem of speech. In writing, we can suss out what someone means if they put it down phonetically. Trust me on this. My older Romanian son still doesn't spell well, but we can read his texts just fine. Spelling is a definite obstacle for an adult learner who wants to look like she knows what she's doing, but it is not an obstacle to understanding. Also, it is often overstated. Most words are phonetic, and even the ones which aren't are usually off on only one sound, with the rest being unimpaired. The main disconnect is in stressed syllables, between how a word looks like it should be accented and how it actually is.
It's going to be English for a long time.
**The Soviet Union kicked this backwards a fair bit. As most movement was considered suspicious and internal travel generally discouraged - except for Russians needing to run stuff - the Slavic languages became more isolated, not less, despite the stated goals of unity. And the Romanians and Moldovans, speaking the only Romance languages in the bunch were really screwed.