The Elvish language Tolkien created for the Sindar is Sindarin.
Sindarin is one of Tolkien’s best-developed Elvish languages in the mythology of Middle-earth. It is associated primarily with the Sindar, or Grey Elves, who remained in Middle-earth rather than completing the journey to Valinor. Tolkien based much of Sindarin’s sound and historical development on Welsh, while Quenya was influenced strongly by Finnish and Latin.
Sindarin and Quenya are not interchangeable names. Quenya is associated especially with the High Elves and has a more formal, ancient status in many stories. Sindarin became a widely used Elvish language in Middle-earth, including among many Elves and some other peoples. Tolkien revised both languages repeatedly as he developed their histories, so published forms can differ across texts and periods of his work.