Primordial

Primordial is said the be the oldest language in existence. It bares no similarities to any modern language, and is even inherently magical in nature - it speaks to a deep, visceral part of a creature's brain, rather than any kind of linguistic structure. It is a truly universal language, able to be understood by all. There is no written form of Primordial.
But with its power comes great complexity - is effectively impossible to speak it without the aid of magic, and incredibly difficult to learn even with its aid. Masters of the language are ancient creatures and gods capable of encompassing what would be hours of mortal speech into a scant few seconds.

In 5th edition Dungeons & Dragons, the "Tongues" spell is effectively translating a user's speech into a pidgin form of Primordial. While not a true replacement for Primordial, it allows creatures to understand each other through language barriers. Primordial is not a language that can be picked as a language choice.