Welsh especially influenced J.R.R. Tolkien’s invented language Sindarin.
Tolkien based Sindarin’s sound and several of its structural qualities strongly on Welsh, a language he admired for its phonology and historical development. Sindarin became the everyday Elvish language associated particularly with the Elves of Middle-earth’s western regions and with many names in The Lord of the Rings.
Tolkien did not simply copy Welsh vocabulary. He constructed Sindarin as a distinct language with its own history, sound changes, grammar, and relationship to other Elvish tongues. In the internal history of Middle-earth, Sindarin developed from earlier forms related to the languages of the Elves.
Finnish is another important influence in Tolkien’s linguistic work, but it is more closely associated with Quenya. Tolkien also drew inspiration from languages such as Old English, Old Norse, Latin, and Gothic. These influences helped create a linguistic texture rather than direct translations of real-world languages.