Re: Abbé Henri Boudet, The True Celtic Language and Cromleck of Rennes-les-Bains (1886).
Abbé Boudet was not the only person claiming to have discovered the original – or primordial – Language of Mankind. Many Antiquarians dabbled in this Pseudo Science that was quite popular in its heyday.
One of the most famous books was the two volume work Anacalypsis: An Attempt To Draw Aside The Veil of Saitic Isis; or An Inquiry Into The Origins of Languages, Nations, and Religions by Godfrey Higgins (1772-1833).
Such things have become obsolete and Abbé Boudet’s book about the Primordial Language is regarded as a “one off” when in fact it was no such thing. It was one of Abbé Boudet’s favourite hobby-horses since he wrote another work on the same topic about Narbonne (Du Nom de Narbonne et Exemples L’interpretation de Mots Gaulois par Les Racines Saxonnes de l’Anglais).
There is also the book by Umberto Eco, The Search For The Perfect Language (Oxford, UK; Cambridge, Mass., USA: Blackwell, 1997, ©1995.).
Abbé Boudet was merely a prudish amateur archaeologist when he wasn’t the vicar of Rennes-les-Bains.
Here is a link to a PDF of the book by Godfrey Higgins.
Here is a link to a PDF of the book by Umberto Eco.