The ancient Elvish language used by the Sindar in Tolkien’s Middle-earth is Sindarin.
Sindarin is one of Tolkien’s principal constructed Elvish languages. It is associated with the Sindar, or Grey Elves, who lived in Beleriand during the First Age. In later ages, Sindarin became a widely used spoken language among many Elves in Middle-earth, especially in regions influenced by the Sindarin-speaking Elves of Beleriand.
Quenya is the major contrasting Elvish language. Tolkien associated Quenya especially with the Noldor and the High Elves, and it often has a more formal or ceremonial role in the stories. Sindarin and Quenya are related within Tolkien’s invented linguistic history, but they are not interchangeable names. Tolkien developed their sounds, grammar, and vocabulary as part of a larger family of Elvish languages, drawing inspiration from historical languages including Welsh and Finnish. Names such as Lúthien, Galadriel, and Mithrandir reflect Tolkien’s linguistic construction.