I'm a bit of a pedant when it comes to writing and, unless there's absolutely no appropriate term in English (for example, schadenfreude) I try not to have untranslated foreign words in my works. I'm working on something inspired by Renaissance Italy and I'm considering translating the civic office of "Gonfaloniere" into "Bannerman" or something similar. Would that lose any nuance that I am overlooking? Is the term "Bannerman" too closely tied with East Asia and the Manchu? Is there another extant English word that "gonfaloniere" could be translated to that's better or more accurate than "Bannerman?"
The word "bannerman" was sort-of used in the sense you intend in English, but that use is quite archaic and is only preserved in surnames. However, the medieval English (and Scots, etc) "bannermen" were a particular kind of military officer, not a civic office held by a politican. Unless you explain in considerable detail what the duties and privileges of a gonfaloniere or "bannerman" are in context, a contemporary reader will probably not understand what you mean.
In a straightforward work of scholarship or other non-fiction, I would just leave it untranslated, but for a work of historical fiction, it's up to you. Possible alternatives might be:
My personal preference is to translate it as either "standard-bearer" or else something descriptive like "chief guild magistrate",
Finally, I'd like to suggest you PM user AlviseFalier, whom is our resident expert on Medieval Italy,