Another lesson we can learn from Sci-Fi is very often different species on a planet would have their tribal / local languages and dialects but all spoke a common tongue. I think this is the more humanizing approach, rather than delegate even more of our fleshly processing power to machines.
Historically, we've seen the larger languages build themselves up by intentionally stamping out the teaching / use of smaller local languages. France banned some regional languages from appearing on broadcast television for years, and etc.
This might be required to get full buy in for a unified language, which is a bit sad but makes some sense - if you ensure it's taking up more and more of media and culture more people know it from immersion, and other languages are reduced to being spoken at home / with friends and that's going to cut into how many people really are fluent in them.
It varies. A lot of local languages have gone extinct already. There's linguists hard at work to try and document / record dying languages, but it won't be the same as living the language from childhood.
