Would the world be better (in any way) if everyone spoke the same (and only) language?

Yes.

But reaching that point would be bloody. Say some federation or single government has jurisdiction over the entire world, and its decided that Chinese shall be the new global language…or Arabic…or Bushman. People would probably start wars over language alone.

Underground speakeasys would pop up, with people speaking forbidden languages…and drinking whiskey. Then yelling in Russian or something.

But assuming it was somehow done, and everyone on earth speaks the new Globalese, created by combing all languages together with weightedness of words and grammar based on population of speakers of each language, then I think the world would be better off. Like you said, cultural richness would be lost though. But the benefits of greater cultural similarity and communication would out weight the costs.

I’d guess English is considered a modern language, but apparently we still don’t have unversal agreement on what the meaning of ‘is’ is.

Civilization would collapse as the masses would abandon all productive labors, instead devoting their energies to their innate urge to build really huge ziggurats.

I think we’d still have everybody trying to correct everyone else’s language. It seems to be a basic human need.

(There must be something wrong with what I just wrote, I suppose.)