Theoretically, some could say if there was only one language in the world, well, the humanity would benefit greatly because the communication would improve, I guess.

However, I highly disagree with any... huh, ideas pursuing monolinguisticism, if I can use such a word(or monoglottery?). Not only the process of acquiring a tongue is beneficial for a human brain - different languages are whole different ways of thinking, perceiving the world! And a tongue is often what forms a base of someone's identity. To get the world to use one language, we need to destroy incredible number of existing tongues which often means destroying cultures, communities, nations... And we would need to decide, which tongue would be 'the chosen one', which clearly leads only to linguistic imperialism.

Also, I write 'we', but in reality the people who would decide would be a very specific group of people from a certain country(or countries). 

It would be a very harmful process. To all. Maybe except some people who have been native speakers of 'the chosen tongue'.

Actually, it is a nice idea for a dystopian novel or so, to think about it. 

i think so life life would be easy ..:)