J.R.R. Tolkien calls the language of Mordor used in the One Ring inscription the Black Speech. Sauron created it for the servants of Mordor, intending it to unify the many peoples under his control.
The Ring inscription begins, “Ash nazg durbatulûk,” translated as “One Ring to rule them all.” It appears in the tengwar writing system, but the writing system and the language are not the same thing. The inscription’s letters are Tengwar, while its language is the Black Speech.
Tolkien supplied very little Black Speech compared with languages such as Quenya and Sindarin. The main surviving example is the inscription on the Ring, along with a small number of names and terms associated with Mordor.
Readers sometimes identify the inscription as Elvish because it uses elegant Elvish-looking letters. That is a mistake: the script is Tengwar, but the words were composed in the language of Sauron and his realm.