Primordial

Script: Unique

Summary: Primordial is the oldest language, so old that it cannot be separated out from the reality that it engulfs. The languages of the elementals (Aquan, Auran, Ignan, and Terran) are all dialects of Primordial, and anyone who speaks Primordial can understand all of them. 

Sounds: Primordial is the sound of a wave hitting the shore, of wind blowing through a valley, of mountains raising and falling, and of the sun burning its way through the sky. 

Appearance: Primordial has no script - it is older than the concept of writing - but it can be seen in the way that the sun shines through clouds, or the way grass moves in the wind.