To start using Infernal in game, type -in console command for Mith's Virtual Console.
Infernal is the common tongue of Baator, of Devils. Any priest or (un)holy warrior who deals regularly with creatures of Baator will learn this tongue as a means of communicating with the lower denizens. Of course also humanoids with Devil blood in them know this language. In short, Infernal is a common tongue of planar, lawful evil.
On Arelith this language is known by: