It’s a case of censorship. Translators' handlings of the various stories ten has varied from leaving the work untranslated, bowdlerizing the stories or—my favorite—one early edition translated the stories, but into French, presumably because readers who understood French were likely already morally compromised.
In his translator’s introduction to the Penguin Classics edition of The Decameron, G. H. McWilliam gives an overview of the history of the translation, non-translation and bowdlerization of the Day 3 Story 10.² He writes,
The treatment of the tenth tale of the Third Day offers a fascinating insight into the English national character as well as providing some indication of the our attitude, as a nation, towards the description and discussion of the sexual act (as distinct from its legitimate or illicit enjoyment). (25–26)³
McWilliams notes that the earliest translations were published anonymously, writing,
it is clear that until comparatively recently a certain stigma was attched, in the puritan English consciousness, to anyone translating so palpably licentious a work as the Decameron, even if the translator took care to write in his preface—as one of them did—that “wherever [he] met with any thing that seemed immodest or loose, [he had] studied to manage the Expression, and conceal the Matter, that the fair Sex may read it without blushing.”
The first two translations (1620 and and 1704) took liberties with the text and omitted entirely two of the day 10 “novels”⁴—story 10 from day 3 and day 9. In 1741, an edition was published (again with an anonymous⁵ translator) which restored the two missing novels and attempted to make the translation more faithful to the original, but still omitted pieces of the restored novels (which were further cut in an 1804 edition of that translation).
The first translation published with an identified translator appeared in 1855, but here the translator, W. K. Kelly said of his work, “The present edition will be found to be COMPLETE, although a few passages are in French or Italian.” (28)
In 1886, a complete unexpurgated edition by John Payne was published, but was only available by private subscription to the Villon Society. And while this edition provides useful commentary through footnotes, the self-consciously archaic style prevented it from becoming the definitive edition of Bocaccio. (28–29).
I’ll leave further investigation into the history of translations to the reader who cares to pick up a copy of McWilliam. The whole translator’s introduction is surprisingly delightful⁶ and I think that his translation is likely the best translation currently available for the contemporary reader.
- Added 11 December 2022 at the prodding of the very persistent Rand al’Thor. As for why it took seven months to do this, I can only paraphrase Samuel Johnson: “Indolence, madame, sheer indolence.”
- The translation is available in multiple editions and is probably the definitive contemporary translation now. I have the Penguin Classics edition (which I bought and read mine in the 80s).
- All page numbers are from the 1987 Penguin Classics edition.
- “Novel” was used early on to refer to any fictional prose narrative, regardless of length. Only in the mid-to-late eighteenth century did the term become restricted to our contemporary understanding of the term, putting aside the tendency of some twentieth and twenty-first-century individuals to apply the term to any book-length piece of narrative prose.
- I should note that McWilliam has provided confirmed or presumed identifications of these translators in his text, but as they’re beside the point of this answer, I won’t bother to give their names. Besides, you wouldn’t know them.
- Usually, translator’s introductions are turgid orgies of self-justification.