Let's say that there was such a meeting and "we" all decided that we're going to speak Klingon or whatever as a lingua franca. How do you think that would work? People wouldn't care what the decreed language is, maybe you'd have UN meetings in it, pass laws to teach it in school in lots of nations or something.
It would probably become even more of an elitist skill than just using a language that already has lots of native and non-native speakers already and lots of culture to go along with it, just like Latin was back when we effectively had this sort of arrangement, sans the constructed language.
Finally, if you really haven't heard a better reason for why English became the current dominant language than "it's already spoken by X people, but how come not Chinese and Spanish" you really owe it to yourself to read some of the things that come up when you Google the likes of "history of lingua franca" etc.