The Elvish language commonly spoken in Middle-earth is Sindarin. Tolkien created Sindarin as the language of the Sindar, or Grey-elves, whose home was Beleriand during the First Age.
By the Third Age, Sindarin had become the everyday Elvish language across much of western Middle-earth. Even many Noldor, who originally spoke Quenya, adopted Sindarin after returning to Middle-earth. It is the Elvish language most often meant when characters refer simply to the “Elf-tongue.”
Sindarin and Quenya are related but distinct. Quenya functioned more like a formal, ancient language, while Sindarin was the more widely used spoken tongue. Tolkien shaped Sindarin’s sound and grammar partly on Welsh, including features such as consonant mutations. It was written first with the Cirth runes and later commonly with the Tengwar script.