Finnish most influenced J.R.R. Tolkien’s fictional language Quenya.
Quenya is one of Tolkien’s Elvish languages and is associated especially with the High Elves. Tolkien was a professional philologist, and he built his invented languages with attention to sound patterns, grammar, vocabulary, and historical development rather than treating them as collections of decorative words.
Finnish was a major inspiration for Quenya’s structure and sound. Tolkien had studied Finnish and admired the Finnish epic Kalevala. He also drew on other languages and traditions, so Quenya is not simply Finnish with substituted vocabulary. Its history within Middle-earth, including its relationship to other Elvish languages, is part of the fictional linguistic system.
Welsh had a particularly strong influence on Sindarin, another major Elvish language. This distinction causes a common mix-up: Quenya is chiefly linked with Finnish, while Sindarin is chiefly linked with Welsh.