The language spoken by the Orcs of Mordor in the One Ring inscription is the Black Speech.
Sauron created the Black Speech for the servants of Mordor, intending it to function as a common language among the forces under his control. The Ring inscription begins, “Ash nazg durbatulûk,” meaning “One Ring to rule them all,” and continues with the lines that identify the Ring’s power.
A common mix-up is confusing the language of the inscription with the writing system used to record it. The inscription is written in Tengwar, a script associated with the Elves, but the words themselves are in the Black Speech of Mordor. The distinction is similar to separating a spoken language from the alphabet used to write it.
Tolkien supplied only a limited amount of Black Speech compared with languages such as Quenya and Sindarin. Even so, the inscription gives the language a powerful role in The Lord of the Rings and connects it directly with Sauron’s attempt to dominate the other Rings of Power.