All the spoken dialogue is in Esperanto. Created by late-19th-century scholars as a "universal language" to unify the world's people, Esperanto has never achieved its goal, and is merely regarded as a fun, popular novelty.
William Shatner grew up in Montreal, Canada, and probably because of this he keeps pronouncing certain Esperanto words as if they were like French. Listen for him saying "sen" (without) pronounced as if it were French "sans", or "sento" (feel) as if it were French "sentir" (to feel). For the record, Esperanto has no nasal vowel-sounds like French does.
In his commentary for the DVD, William Shatner recalled an incident that occurred when the cast and crew first arrived in Big Sur, California. He remembers a "hippie" man approaching the company, and inquiring into their endeavor. Shatner says that the cast and crew reacted with some hostility to his interest, which angered him in turn. The "hippie" then loudly put a curse on their production, which some people believe came in effect.
After this film generated acclaim in film festivals around the world and in successful theatrical showings in France, a lab mistakenly destroyed the negative and all prints; the film was considered lost and never released on video. Then, after years of searching, one print was discovered in the permanent collection of the Cinémathèque Française in Paris. From that sole surviving print, this film has been digitally restored with remastered sound. It has since been re-circulated online, generating a new fan base.
According to Anthony M. Taylor on one of the DVD commentary tracks, the screenplay was written in English and translated to Esperanto. When the sole surviving French print of the film was remastered for release, English language subtitles were created based on the original English language screenplay, not from a translation of the spoken Esperanto dialogue back into English. This results in as strange situation for a foreign language film: discrepancies between the English language subtitles and the spoken dialogue are actually due to mistranslations from English to Esperanto, not vice versa, or else reflect late changes which were not back-ported to the original English language screenplay.
In Esperanto, an Incubus (a male demon who seduces women) is "inkubo" (een-KOO-bo). But when Arndis says she's had a nightmare, nightmare in Esperanto is also "inkubo", or preferably "inkubsong^o" (een-koob-SON-jo).