Tolkien did a lot of work on Quenya, Sindarin and other languages for middle earth. None of them were in a state where they could be considered working languages, however.
I hear Dothraki was pretty well developed. I've no idea.
Firefly and the Expanse have both developed a kind of multi-language patois they can lapse into when the scene needs it, without having to really develop it, but they're probably some of the most realistic.
The best developed constructed language used for a fantasy movie of course had to involve..
SHATNER
Incubus used Esperanto as its language for wherever the movie was supposed to take place. It works pretty well. I havent taken an esperanto course in years so I cant understand a word of it now.
Frank Herbert did not fully develop Chakobsa, but did more work with it than was typical of the time he started writing (lets be honest, world-building would not be what it is without him)
but there is enough Chakobsa that the Children of Dune miniseries was able to put together a Fremen song.
I think it's pretty good.
