Modern French belongs to the Romance branch of the Indo-European family, descended from the Vulgar Latin of Roman Gaul, and it is the language in which a large part of the modern Western esoteric tradition was first written.
The nineteenth century French occult revival is the clearest case. Eliphas Levi wrote his systematic treatments of ceremonial magic and of tarot symbolism in French in the 1850s and 1860s, and English-language occultism met them only once they had been translated, decades later and through translators with purposes of their own. The tarot of the Golden Dawn, and through it most of the twentieth century's, rests on a French foundation reached at one remove.
Behind that revival lies an older French grimoire literature, magical books compiling ritual instructions, invocations and tables of correspondence, of which the Grimorium Verum and its seventeenth and eighteenth century companions are the best known. Those in turn draw on Latin and vernacular manuscript traditions, so the French is itself a layer rather than a beginning.
French Catholic and Huguenot Protestant theological writing forms a third body, and the same question applies to all three: whether a given text was written in French or arrived in it. The answer changes what a disputed phrase is evidence of, and it is a fact about the text rather than about the person reading it.