Undercommon

Undercommon is the language used in the Underdark. It is a blend of Drow, Dwarven, Common, and Elven. There may be some words from other languages thrown in from time to time. To someone from the surface who has never heard the language, it may seem like a multilingual person is throwing together words from all of the languages they speak, however, it is intentional and the language is uniform across the Underdark.
Undercommon uses the Elvish script and most speakers also learn to read and write.