The Johnson wrote:
Does everyone on earth speak English in the star trek future?
-- This is just my opinion, feel free to politely disagree --
English is already one of if not the most widely spoken language on Earth already. I assume that there are still other languages used by humans living wherever but for the sake of simplicity, representatives of Earth speak English.....
I think that it would similar to a local / central government style in that there would still be national governments deciding things, but any major changes that will affect how the whole planet interacts with other species are decided by the Planet Government.....
As proved by Picard / Chakotay, national culture is still very important and defines how people live but they can still get along with others despite any cultural differences.
This basically says it all...