Infernal

The language of demons has not been heard since the dawning days of the world. Demons first came into existence as aspects of the Infernal Realm, much as faeries are manifestations of the Faerie Realm, and there was a time - when it was and how long it lasted is taught differently by every religion - that only the demons lived there. Then, though far, far from friends to humanity, or anything else, the demons were not as concerned about affairs in the mortal world, or the other Realms, and while they did spread ruin where they went, and encouraged those they encountered to sin, there was no end goal to their malice, nor broader plan to enact it.
This changed with the coming of The Adversary. After its emergence - demons happily spread misinformation about where it came from and why, and different religions offer different versions of the story - the Adversary was quick to enslave the demons and convert them into a weapon in its war with existence. Much of the Infernal Realm was destroyed, the Adversary permitting only things that could be used for destruction and suffering to remain intact, and one of the things that was lost was the language of the demons. The Adversary had no use for it - demons only need the languages of their victims for his purposes, so it was erased: the Adversary hates demons no less than any other part of creation, after all, and wishes the destruction of all things, physical, mental, and spiritual - even the spoken word. 
Anything written in Infernal degrades faster than it should - books go to damp and mold wherever they are, and etched stones or metal abrade away in a process with no natural origin. This can be stopped if the writing is kept within a Divine aura, though only as long as it remains there, and slowed by the presence of a Magic aura. Faerie auras speed it up or slow it down unpredictably.

Modern Understanding

Little of Infernal survives to the present day, and ironically what does is usually found at old sacred sites. Churches, temples, towns and cities also have Divine auras that could feasibly maintain Infernal script, although the death of the language predates them so any found there has been moved from some other site. There are no speakers of Infernal, native or otherwise, but a handful of words have been translated and a few sentences can be understood in context. Usually, where the script is preserved, it is through a lack of understanding what it is. Efforts to decipher or preserve the script have been hampered by most who learn what they have destroying it out of fear that it may taint their souls. Scholars seeking out Infernal script are also often mistaken for infernalists and must disguise their intentions in civilised places. For example, there exists a proscribed mystery cult, known as the Conversants, who seek out traces of the language. Their successes, and purposes, are known only to members, although it is known that their interest is in languages generally and not just in Infernal. Even so, priests are naturally suspicious of people asking about demonic language, and more than a few have been killed by mobs of angry peasants or townsfolk.