Abyssal

Some believe that Abyssal was the first language developed by the inhabitants of the Outer Sphere, although this is very difficult to prove given its incredible rate of linguistic change. Abyssal is the native language of the demons of the Abyss, although it is also spoken by many devils, daemons, or others who have frequent dealings with races of the lower realms.
Abyssal is spoken on Golarion in the Worldwound, a region in northern Avistan with a direct connection to the Abyss that is overrun by demons.