Reading about the origin of the phrase “lingua franca” makes me wonder what it would be like to run a fantasy RPG campaign in a setting where the “common tongue” is really just a pidgin trade language with limited utility, so actual knowledge of different cultures’ languages is no less beneficial than skill with weaponry.
@JasonT That's basically the assumption I made in my Owl Light setting. Most people understand the literal "Trade Tongue". Is pretty useful for commerce, and *can* communicate many other ideas. But most folks don't feel it is very evocative or descriptive for other purposes.
You'd usually use specific other languages if you wanted to write a poem, explain an engineering process unambiguously, encode a spell, describe transcendent reality, cuss someone out, fit in with the local population, etc.