The language in Tolkien’s One Ring inscription is the Black Speech. Sauron created the Black Speech for the servants of Mordor, and the Ring inscription is its best-known surviving example.
The inscription begins, “Ash nazg durbatulûk,” translated as “One Ring to rule them all.” It was written in Tengwar script, but the script and the language are different things. Tengwar is a writing system, while Black Speech is the spoken language represented in this passage.
Readers sometimes identify the inscription as Quenya or Sindarin because those Elvish languages are more prominent elsewhere in Tolkien’s legendarium. The inscription’s language is instead associated with Sauron and Mordor. Tolkien supplied only a small amount of Black Speech, so it is far less extensively developed than his Elvish languages.