J.R.R. Tolkien used Finnish as the main model for his invented language Quenya.
Quenya is one of the best-developed languages in Tolkien’s legendarium and is associated especially with the High Elves. Tolkien drew inspiration from several real languages, but Finnish strongly influenced Quenya’s sound system, grammar, and overall aesthetic. He had studied Finnish and admired the Kalevala, Finland’s national epic.
Quenya was not intended to be Finnish in disguise. Tolkien created its vocabulary and fictional history, placing it within the Elvish language family of Middle-earth. Its related language Sindarin was influenced more strongly by Welsh, while other linguistic ideas contributed to the broader structure of Tolkien’s linguistic invention.
Readers sometimes identify Welsh as the model for all Tolkien’s Elvish languages. That is too broad: Welsh was especially important to Sindarin, whereas Finnish is the standard answer for Quenya’s principal real-world inspiration.