Thought Experiment: World language
If everyone in the world spoke every single language all of a sudden, do you think we would converge on a single mostly used language?
I've been in groups where most people spoke several languages sufficiently fluently not to care (and in some cases not to know!) which they were speaking. Every time this has happened the language in use changed according to the topic being discussed, sometimes in mid-word. Sometimes one language just makes it easier to talk about something than another.
In one book I have read it says something along the lines of:
One speaks of love and such trivia in French, of food
in Italian, of tragedy in Russian, does business in
German, and addresses the servants in English.
Similarly, I regularly program in AWK, Python, C, C++, and ARM assembler, and I dabble in Lisp, Lua, OCaml and Haskell. In each case I reach for my most comfortable tool for what I'm working on, and it's not always the same one.
So I think if everyone were equally fluent in every language, there would be a pidgin arise, a language that is a mixture of the others, possibly with sub-languages for specific subjects.
Lets say you need around 3,000 words for a normal day to day conversation (though I suspect that it might be higher). That 3,000 words in Chinese, 3,000 words in German, 3,000 in Farsi. You get the idea. There has got to be more than 2,000 living languages in the world - admittedly some of them have only a handful of speakers - so you will need to remember 6,000,000 vocabulary items.
That alone is a nightmare but remember that not all words have direct translations from one language to another, English does not distinguish gender the same way that Arabic does. Japanese has different number words for different types of objects. So where some languages use only one vocabulary item others will require several.
English has, as I understand it, the largest vocabulary with more than 300,000 items (we didn't just steal people countries when we built the empire we stole their words too). Take it from me, no one knows them all.
6,000,000 - you don't stand a chance.