Pardon my wall of text:

I’m blowing smoke on this topic based on dnd terms. Common tongue is English because more people speak that already. Whatever people do to it after, they do to it after, but English as a starting point removes the need for more people to learn Esperanto or whatever other language wants to be “common.”

In the fantasy I’ve created in my head/my hot take: it would be easier if people shifted to English as common, and had “secondary” languages based on their location.

(In the real world, even sign language isn’t universal. Certain languages like German apparently also do better for certain things like written law. I realize it’s silly to expect our species to choose a common language based on numbers alone. I think more people speak English specifically because English speaking countries hold more sway on international economics/warfare (for now), and other countries have responded in kind.)