Months before Star Trek, William Shatner starred in a horror film performed entirely in Esperanto, the constructed international language created in 1887 by a Polish eye doctor.
Not one person on the production actually spoke Esperanto. The cast learned their lines phonetically in a couple of weeks, with no fluent speaker on set to correct them. Supposedly, when the film premiered, a crowd of Esperanto enthusiasts laughed out loud at the pronunciation.
Writer and director Leslie Stevens chose Esperanto not because he believed in the ideal of a shared global language, but because he wanted an otherworldly feel and a distinctive selling point.

The language gimmick tends to be all anyone hears about INCUBUS (1966), which is a shame, because underneath it is a beautiful, eerie film.
Stevens, who was the creator of The Outer Limits, brought his television crew with him. The black-and-white photography was by Conrad Hall, who would go on to win three Academy Awards. His camera operator was William Fraker and his assistant was Jordan Cronenweth, who would later shoot BLADE RUNNER.
The result, a stark folk-horror tale of a succubus who sets out to corrupt a pure-hearted soldier and undoes herself by falling in love, looks less like a curio than a lost Bergman filmed on the cliffs of Big Sur.
Incubus nearly vanished for good. A lab fire destroyed the negative and every print. For thirty years the film was simply gone, until one damaged copy surfaced in a Paris archive, French subtitles burned into the frame. A better 35mm print was discovered in 2023, also with burned-in French subtitles. Some modern prints crop, obscure, or find other creative ways to replace the subtitles.