I think it would be a disaster for this site and all the teachers here!
Actually, learning a language is a good way to interact with others, it is said to be a great way to avoid problems like dementia, different languages reflect different cultural priorities and ways of reflecting on life.
One language would mean everyone could communicate but it would impoverish that communication.
Ìt isn't impossible but it would be tiresome. The world is beautiful because of its diversity in almost everything. It's the way nature is.
I can't imagine that. With lots of languages we know each other better, through diversity.
Suppose everything in the world was red or grey. Ouch, how monotonous!