The French translation of the litany does refer to it by the same phrase. As much as Americans like to imagine French people are all very poetic, in everyday conversation the word French people use to describe an orgasm is “orgasme” so there would be no confusion about the meaning. “Petite-mort” used to refer to fainting—another temporary weakness/unconsciousness—but it’s not an everyday phrase. I’m not a native speaker but I lived in the Swiss part of France for a while and dated many native speakers, and they all exclusively used the more literal term in a sexual context. I’ve always assumed that Herbert used it intentionally—the Reverend Mother who puts Paul Atriedes through the ordeal where he says the litany speaks an old French-derived language, so it makes sense that a French phrase would creep into the text.

Edited: If you’re curious, the text I have is below. It doesn’t have the exact same cadence as the original, but it retains the same sentiment, and has a style of its own. Litanie contre la Peur

Je ne connaîtrais pas la peur, car la peur tue l'esprit. La peur est la petite mort qui conduit à l'oblitération totale. J'affronterai ma peur. Je lui permettrai de passer sur moi, au travers de moi. Et lorsqu'elle sera passée, je tournerai mon oeil intérieur sur son chemin. Et là où elle sera passée, il n'y aura plus rien.

Rien que moi.