In his 1988 book Rennes-le-Chateau: Le dossier, les impostures, les phantasmes, les hypotheses, de S.de affirmed that Boudet wrote La Vraie Langue Celtique in a cryptic fashion, adding that he used Jonathan Swift's A Discourse to Prove the Antiquity of the English Tongue, Showing from various instances, that Hebrew, Greek, and Latin were derived from English (1712) as a guide. Here is the work by Swift for your perusal ....
https://www.rhedesium.org/articles/of-the-antiquity-of-the-english-tongue?c=rennes-les-bains