The Common Tongue in Fantasy Fiction

The common tongue is not a terribly realistic trope, as it's presented in a lot of fantasy fiction. If there is something that approximates a quote unquote common tongue, it is going to be the result of some kind of imperialism usually. So this idea that you would have a lingua franca that everyone knows and there's no barriers to understanding linguistically may be convenient for some kinds of stories, but it's not terribly realistic.