D&D 5E Using Languages like Tools

[Greenstone.Walker]

If Common bugs you, then replace it with Trade (which is what I have done in my current game).

Trade is simply a language for commerce. You can greet people, buy and sell, and talk about the weather and the dangers of caravan routes, but it has no culture. You can't discuss philosophy or religion or science in anything except the most general terms. No sonnets, no operas. It doesn't have metaphors.

Everyone speaks Trade.
Everyone also speaks one or two languages from where they grew up (which for humans might be Damaran or Illuskan, for someone who grew up on the Sword Coast of FR).

 

[Li Shenron]

I like the idea of using language proficiency for additional uses.

OTOH I would not spend a minute of work to try and figure out how to give the same to Common, because it already has the advantage of being spoken 1000 more times than any other language, and every PC speaks it anyway.