The mystery is why the French version kept the Japanese names instead of using the French names, which they actually did with all the other cities.
Wait, what?
EDIT:
'Kay, I'll just explain the whole thing right now, as I don't imagine a lot of people in this thread are particularly interested in French translation choices...
So that Tumblr said:
Overall I’m really happy with the French translation, even though it’s an older edition and it shows in a few romanization mistakes (Formaggio is written as “Hormaggio” and Naples - or Napoli in Italian, since they write the names in Italian - becomes “Neapolis”? While all the other cities are written properly? It is a mistery).
First off, "Hormaggio" / "Formaggio"...
In Japanese, that character's name is spelled "ホルマジオ" ("horumajio"). And since (I'm pretty sure) there was no official alphabet spelling for that name at the time of the translation, you kinda had to deduce said spelling.
So. A lot of Part 5 characters are named after Italian words for various types of food, and here, we have something extremely close to "フォルマジオ" ("forumagio"), the Japanese transliteration of "formaggio", the Italian word for "cheese".
So while there's little doubt as to what the name is based on, there's an ambiguity as to how it should be spelled in alphabet: did Araki use "ホ" ("ho") instead of "フォ" ("fo") on purpose, or was that a mistake? There would be precedents for both possibilities, so it's a bit of a coin toss between acknowledging or ignoring that odd "ホ" ("ho") in the alphabet spelling.
Since Araki is supposed to be fairly familiar with Italy and Italian food, I reasoned he
probably wouldn't make a mistake and went with the former, although I admittedly had no reason to feel particularly confident about that choice.
Years later, the
JoJoveller artbook went and used the alphabet spelling "Formaggio" (I'm under the impression it simply used the same name spellings as the Italian translation of the series, for Part 5), so if I had to to do it again,
that's what I'd use, now. But there really was no way to be sure, at the time.
Anyway, as said above, I'm a bit surprised someone who seems fairly knowledgeable wouldn't be aware of the original "ホルマジオ" ("horumajio") spelling and/or wouldn't see where I was coming from.
As for "Neapolis" / "Napoli" / "Naples"...
That author of that Tumblr apparently believes Araki used the Italian name of "Naples", i.e. "Napoli" (well, that would be "ナポリ" ("napori"), in Japanese) in the original text. That seems like a fair assumption considering, well, that
is how you say "Naples" in Japan.
Thing is, he didn't. He called that first city "ネアポリス" ("neaporisu"). And we can catch a glimpse of a "NEAPOLI-" and a "NEAP-" (in alphabet) in a wallet in volume 48, page 71, when Giorno first meets Polpo.
So, yeah, Araki apparently went with "Neapolis", i.e. the old name of Naples.
My personal theory as to what happened there (I should have confirmed that with Araki when I had the chance, in retrospect)... I think Part 5 and the Passione gang were originally supposed to be much smaller in scale (note how the
one arrow the organization had was in Neapolis
specifically, there was nothing in Bucciarati's volume 48 explanations that hinted at Passione extending beyond that city either, and in that same volume, Giorno was talking about defeating the boss and taking control of
Neapolis). Much like Part 4. And just as Araki used an obviously-based-on-a-real-place-but-ultimately-fictional town in Part 4, I think that's what he intended to do in Part 5. Hence "Neapolis": a fictional version of Naples. But then, Araki probably figured it would be a waste not to use this golden opportunity to travel around in Italy and do some sightseeing, so he switched to another approach more akin to Part 3's, real place names and all. (Note that Araki used "Neapolis" again in Part 7, where it was the name of a fictional kingdom.)
So, there again, as knowledgeable as that Tumblr person appears to be, it seems he/she claimed "Neapolis" was a romanization mistake without actually checking the original Japanese beforehand.