Today's has an article on "Incubus," which is probably the
only full-length film (a fantasy horror story about demons) where the
dialogue is entirely in a conlang.  To wit, Esperanto.

Even better, it starred William Shatner.  Before Trek.

Hmm...  Incubus, Esperanto...  Probably the next major film to feature a
conlang was the Trek movie which first used Okrand's Klingon, and which
also starred Shatner.  (I forget which one actually had Klingon in it)...

Eerie coincidences.

I'm waiting for the Lojban version of Buckaroo Banzai, myself. :)