Why are "Ser" and "San" left untranslated to "Sir" and "Saint"?

Question

Day 1 Story 1 of the Decameron is about the knavish Ser Ciappelletto, who after his death becomes reputed as the holy San Ciappelletto. In the Rigg translation which I'm reading, these names are left untranslated: Ser is not changed to Sir, and San is not changed to Saint. Why are these titles left in Italian in the translation? Do Ser and San carry some Italian contextual baggage that would be lost in Sir and Saint, or is it just for flavour?

Ser Ciappelletto cheats a holy friar by a false confession, and dies; and, having lived as a very bad man, is, on his death, reputed a saint, and called San Ciappelletto.

Ser Cepparello con una falsa confessione inganna un santo frate e muorsi; e, essendo stato un pessimo uomo in vita, è morto reputato per santo e chiamato san Ciappelletto.

(It's also interesting to note that his name was originally rendered differently in two parts of the same paragraph, and this is not preserved in the English translation. The variations of his name, however, are explained in the story, and I've also asked a related question about that.)

Answer 1

The introduction to Wayne A. Rebhorn's translation (W. W. Norton, 2016) contains an explanation about "Titles and Form of Address" on pages xlvi–xlvii of the introduction. This includes an explanation about the use of "Messere (Ser)":

Messere (Ser): This was the honorific title used for a man with something like the equivalent of upper- or middle-class status (ser is short for messere); it was like addressing someone as sir, but without the implication of aristocratic status. Because there was no concept of a middle class in Boccaccio's time, translating it as "Mister" would be anachronistic, and I have accordingly retained the Italian terms. (…)

Rebhorn does not discuss "San" and translates "san Ciappelletto" as "Saint Ciappelletto".

J. M. Rigg may have had similar reasons for retaining "ser" and may have kept "san" untranslated for reasons of consistency.