I think we're pretty close with English, or it's at least the best candidate. Most of the knowledge recently produced (since WWII) is in it, and the world's largest economy uses it. It's also taken a little something from every one.
With that said, I think national pride will prevent such a thing happening. As an American, I wouldn't want to learn someone else's language as a "common tongue", though have no objections to using it for communication. Same thing with units: I'll convert if needed, but won't use yours. I've also got a bit of an objection to "design by committee" (i.e. esperanto) because it is either influenced primarily by one language (which goes back to the "other guy's language" objection above) or has little influence from other languages (meaning massive barriers to adoption). Most Americans are also highly resistant to bureaucrats dispensing "what's best" from on high, be it with language, units, or most other things.
It's also a cultural thing: how much tradition, history, and lore would be lost if we all spoke the same thing?
It will depend on how long the US remains the reigning power. If China bowls the US over, English can be undone fairly quickly. Mandarin is spoken by 20% of the world population.
English is not the obstacle; the Latin alphabet is.
There is a much higher chance that Western countries could move from English to another Latin-alphabet language (e.g. Spanish) than to any language rooted in ideograms. Which is why, for example, Japanese has made no real inroads despite 40 years of massive economic influence.
plenty of overseas chinese in countries that don't speak english... chinese speakers of mandarin and indonesian, minnan and indonesian, mandarin and french, mandarin and german, mandarin and quebecois, etc.
Mandarin is spoken overwhelmingly by residents of China. English is an official language of education and government in at least one country on every continent. In South Asia it’s common for the upper middle classes and up to speak English better than what’s theoretically their native language. If the US was magically replaced by parallel universe with no people US English dominance would last at least a century. Spoken Latin and French both survived as lingua franca long past the apex of their sponsoring powers.
With that said, I think national pride will prevent such a thing happening. As an American, I wouldn't want to learn someone else's language as a "common tongue", though have no objections to using it for communication. Same thing with units: I'll convert if needed, but won't use yours. I've also got a bit of an objection to "design by committee" (i.e. esperanto) because it is either influenced primarily by one language (which goes back to the "other guy's language" objection above) or has little influence from other languages (meaning massive barriers to adoption). Most Americans are also highly resistant to bureaucrats dispensing "what's best" from on high, be it with language, units, or most other things.
It's also a cultural thing: how much tradition, history, and lore would be lost if we all spoke the same thing?