Who speaks it:Fictional characters of Middle Earth (exact number unknown).
What makes it unique:Quenya and Sindarin come from the Elvish language family invented by Professor John Ronald Reuel Tolkien in his books “The Hobbit” and “The Lord of the Rings” trilogy.
Long before his books hit the big screen in movie form, avid readers, Tolkien aficionados, and even linguists and scholars undertook serious study of these invented languages.
This study has in turn led to spirited debate: Tolkien worked on developing the languages as a hobby years before the books were published, and he’s known to have repeatedly tinkered with Elvish's structure and content. Consequently there’s no agreement as to whether the published form is the correct one.
What is certain is that Quenya is a form of high Elvish, also referred to as Latin Elvish, while Sindarin, also called Grey Elvish, is the everyday language of the elves.
Interestingly, of all the languages on this list, these ones probably have the best bet of surviving for a long time, due to a thriving fandom around Tolkien's work.