J.R.R. Tolkien based Sindarin phonology partly on Welsh.
Sindarin is one of Tolkien's constructed languages for the Elves of Middle-earth. Its sound patterns and some of its design principles were influenced by Welsh, a language Tolkien admired. He also connected Sindarin historically within his fictional world to earlier Elvish languages.
Sindarin is the language associated especially with the Grey-elves and became the most widely spoken Elvish language in Middle-earth during the period described in The Lord of the Rings. Quenya, another major Elvish language, was influenced more strongly by Finnish and Latin in aspects of its development.
Influence does not mean that Sindarin is Welsh in disguise or a direct translation system. Tolkien created its vocabulary, grammar, sound changes, and fictional history himself. The comparison concerns artistic and linguistic inspiration rather than identity between the two languages.