What language is the Elvish language from "The Lord of the Rings" based on, and who created it?

Tolkien created several languages for his fictional Middle-earth, with the Elvish languages, Quenya and Sindarin, being the most prominent. Quenya, also known as High Elven, was the language of the exiled Noldorin Elves. It was primarily based on Finnish and, to a lesser extent, influenced by Latin and ancient Greek. Sindarin, or Grey-elven, was spoken by the Sindar Elves and was mainly based on Welsh, with some inspiration from other Celtic languages. Both languages come with their own written script: Tengwar for Quenya and Cirth for Sindarin.