Sindarin is the Elvish language spoken by many Elves in Tolkien’s The Lord of the Rings.
Sindarin developed from Tolkien’s ideas about a language used by the Elves of Beleriand. Within the fiction, it became the common Elvish language of Middle-earth, especially among the Sindar and many other Elves in the Third Age. Place names and personal names such as Rivendell’s Elvish name Imladris reflect Tolkien’s linguistic design.
Tolkien based aspects of Sindarin’s sound and structure on Welsh, while Quenya drew strongly on Finnish and other influences. The two languages are related inside the legendarium but are not interchangeable. Quenya often functions as a formal or ancient language, whereas Sindarin is more widely heard in Third Age narratives.
Khuzdul is the language of the Dwarves, and the Black Speech is associated with Sauron. Both may appear in discussions of Middle-earth languages, but neither is the answer to this question.