Welsh was a major inspiration for the sound and style of Tolkien’s Sindarin language.
Sindarin is one of the principal Elvish languages in Tolkien’s legendarium. It is associated especially with the Sindar, or Grey Elves, and becomes a common Elvish language in much of Middle-earth. Tolkien shaped its phonology and appearance partly in response to his admiration for Welsh.
Tolkien was a professional philologist with a lifelong interest in how languages change and resemble one another. He did not simply copy Welsh vocabulary into Sindarin. Instead, he created historical relationships, sound changes, grammar, and vocabulary for the fictional language, giving it the impression of a language with a long internal history.
Finnish is more strongly associated with Quenya, another major Elvish language. That distinction is a frequent source of confusion: Welsh influenced Sindarin, while Finnish was an important inspiration for Quenya.