Sindarin is the Elvish language in Tolkien’s legendarium that was influenced partly by Welsh.
Tolkien constructed Sindarin for the Elves of Middle-earth, especially the Sindar, or Grey-elves. Its sound patterns and some of its grammatical ideas were influenced by Welsh, one of the real-world languages Tolkien studied and admired. Sindarin became the commonly spoken Elvish language of many Elves in the Third Age.
Sindarin is often confused with Quenya, another major Elvish language. Tolkien modeled Quenya partly on Finnish, while Sindarin’s strongest external inspiration was Welsh. The languages are related within Tolkien’s fictional history, but they are not interchangeable: Quenya is associated especially with the High Elves, whereas Sindarin became more widely used in Middle-earth.