Welsh is the principal linguistic basis of J.R.R. Tolkien’s fictional language Sindarin.
Sindarin is one of the Elvish languages Tolkien created for the peoples of Middle-earth. It is associated especially with the Sindar, or Grey Elves, and is widely spoken by Elves in The Lord of the Rings. Tolkien shaped its sound and appearance using features that reminded him of Welsh, including characteristic consonant patterns and mutations.
Sindarin was not simply copied from Welsh. Tolkien constructed it as an independent language with its own vocabulary, grammar, history, and relationship to other languages in his fictional world. Its earlier form, Common Telerin, is part of Tolkien’s imagined development of Elvish languages.
Finnish was especially important to Quenya, another major Elvish language. Old English, Latin, Greek, Gothic, and Old Norse also influenced Tolkien’s language-making and scholarship. Because of these multiple sources, describing Sindarin as “Welsh” is useful shorthand for its strongest external inspiration, not a claim that the two languages are the same.