J.R.R. Tolkien based Sindarin mainly on Welsh phonology.
Sindarin is one of the principal Elvish languages in Tolkien’s legendarium. In the fictional history of Middle-earth, it was associated especially with the Sindar, or Grey Elves, and became a widely used language among Elves in Beleriand and later regions.
Tolkien was a professional philologist and designed his languages with historical relationships, sound patterns, and changes over time. Sindarin was influenced chiefly by Welsh, while Quenya drew strongly on Finnish and also reflected elements of Latin and Greek. These influences concern linguistic structure, not simple word-for-word borrowing.
Sindarin is often confused with Quenya because both are prominent Elvish languages. Quenya is associated with the High Elves and has a different fictional history and sound system.