I'd love to use languages, but my players all believe their characters can speak and read and write every single language, even the weird ones.
They also think that when I say they don't understand a creature then that means they do understand the creature.
Me: The creature speaks in a language you don't understand.
At least one player: Is it Abyssal?
Me: Do you speak Abyssal?
Player: Yes!
Me: Well, given that it's a language you don't understand then its clearly not Abyssal.
Another player: Ooooo, is it Primordial?