The fictional language Tolkien developed alongside Sindarin is Quenya.
Quenya is one of the best-developed languages in Tolkien’s mythology. He modeled aspects of its sound and structure on languages including Finnish, and he used it as an ancient, formal language associated especially with the Elves known as the Ñoldor. Sindarin, by contrast, became the everyday Elvish language of many characters in Middle-earth.
Tolkien was a professional philologist who treated language invention as a serious creative activity. He created vocabulary, sound systems, grammatical patterns, and historical relationships between his languages, rather than merely inventing isolated words.
Quenya is sometimes called “High-elven” or “Elven-Latin” in descriptions of its cultural role. It should not be confused with Sindarin, which Tolkien also developed extensively, or with generic fan-made “Elvish” vocabulary. The languages appear in names, poetry, inscriptions, and ceremonial speech throughout Tolkien’s legendarium.