In 'The Lord of the Rings' the Elvish language Sindarin is based on what real language?

Tolkien based the sound and some of the grammar of Sindarin on Welsh, and Sindarin displays some of the consonant mutations that characterize the Celtic languages.

Sindarin is a fictional language devised by J. R. R. Tolkien for use in his fantasy stories set in Arda (popularly called Middle-earth). Sindarin is one of the many languages spoken by the immortal Elves, called the Eledhrim or Edhellim in Sindarin.

In the Third Age (the setting of The Lord of the Rings), Sindarin was the language most commonly spoken by most Elves in the Western part of Middle-earth. Sindarin is the language usually referred to as the elf-tongue or elven-tongue in The Lord of the Rings. When the Quenya-speaking Noldor returned to Middle-earth, they adopted the Sindarin language. Quenya and Sindarin were related, with many cognate words but differing greatly in grammar and structure. Sindarin is said to be more changeful than Quenya, and there were during the First Age a number of regional dialects.