J.R.R. Tolkien said Welsh especially influenced the sound and appearance of Sindarin, one of his Elvish languages.
Sindarin is the language of many Elves in Tolkien’s legendarium and is particularly associated with the Sindar, or Grey Elves. Tolkien was a philologist who studied how languages change, and he designed his invented languages with historical relationships, sound patterns, and word formation rather than treating them as simple collections of fantasy names.
Welsh provided an important model for Sindarin’s phonetic character and written appearance. Tolkien also drew on other languages and linguistic traditions, so Sindarin is not Welsh with its vocabulary replaced. Its grammar and history belong to Tolkien’s fictional world.
Sindarin is often confused with Quenya, another major Elvish language. Quenya’s sound was strongly influenced by Finnish and Latin, while Sindarin was shaped more prominently by Tolkien’s appreciation of Welsh. Both languages developed through Tolkien’s invented history of Middle-earth.