J.R.R. Tolkien’s fictional language Sindarin was inspired chiefly by Welsh.
Sindarin is one of the major Elvish languages of Tolkien’s legendarium and is associated especially with the Sindar, or Grey Elves. In the imagined history of Middle-earth, it became the everyday Elvish language of many Elves in Beleriand and later remained widely used in Middle-earth.
Tolkien was a philologist with a deep interest in historical language change. He modeled Sindarin’s sound and structure largely on Welsh, while also drawing on ideas from other languages. Its relationship to Quenya within the fiction resembles the relationship between different historical languages: both descend from an earlier language, but they developed along separate paths.
Sindarin is sometimes confused with Quenya, another major Elvish language. Quenya was influenced strongly by Finnish and Latin, whereas Sindarin is the Welsh-associated language. Tolkien created extensive vocabulary and grammar for both, though neither was intended to function as a complete modern language in every practical respect.