But "losing a language" (losing all the users except those studying it) could lead to some loss of the convey of certain unique culture (that can only be represented accurately in that language)
Having one language only is either (1) A group of people forcing every people in the world using it or (2) All people in the world are willing to give up using all other languages
For (1), it's not what myself and many people want as it means our freedom is taken away.
For (2), I think it's unlikely to happen as different languages have different specialties in representing some concepts. Having one language only probably means some concepts cannot be well communicated.
I can speak Cantonese (Hong Kong version), English and read some Japanese (enough for my games & comics). I would never give up any of them.