Welsh most strongly influenced J.R.R. Tolkien’s invented language Sindarin.
Sindarin is one of the principal Elvish languages in Tolkien’s legendarium. Its sound and structure were deliberately developed with Welsh as an important model. Tolkien admired the musical qualities of Welsh and used features such as consonant mutations to give Sindarin a distinctive historical and linguistic character.
Sindarin is not Welsh with Middle-earth vocabulary substituted into it. Tolkien created its own vocabulary, history, and relationship to other Elvish languages, including Quenya. In the fiction, Sindarin became the everyday language of many Elves in Middle-earth, while Quenya had a more formal and ancient status in several contexts. Finnish was especially important to the development of Quenya, so it is a frequent but incorrect answer to a question about Sindarin’s strongest modern-language influence.