These are recommendations for dealing with Italian placenames when translating into or writing in English. Preferred forms are shown in bold.
The following cities should generally be referred to by their English names:
Elsewhere use the Italian; "Mantua" and "Padua" for Mantova
are 50/50, but I would tend to go with the native forms. Syracuse
are definitely obsolete, as is Trent
except when referring historically to the Council of Trent
or artistically to Raw
and Burnt Sienna
pigments for paint. For names of provinces
(which are all named for their principal towns) it is preferable to use the Italian names even in the cases listed above.
NB that In general, in the bilingual Südtirol/Alto Adige region the Italian names (Bolzano, Dobbiaco, Stelvio pass) are preferable to the German ones (Bozen, Toblach, Stilferjoch) but "Brenner pass", not Passo di Brennero). Mountain passes into France and francophone Switzerland are usually better known by the French names (but "Great/Little St Bernard")
A few streets, squares and buildings and other features have traditional English names - the general rule is only to consider translating them in Rome ("Spanish Steps" for Piazza Navona) and Venice (Grand Canal, St Mark's Square), not elsewhere (the Ponte Vecchio and the Uffizi, not the Old Bridge or the Offices!). Parks are usually left in Italian ("Parco Sempione") but "giardini" is translated (the Boboli Gardens, the gardens of the Villa Farnese) Churches are almost universally referred to by their Italian names with the exceptions of St Peter's (San Pietro) in the Vatican and sometimes St Mark's (San Marco) in Venice. Use the Latin name - the Colosseum, the Forum, Herculaneum - for Roman remains and sites (the Italians normally translate them) except "Appian Way" for Via Appia. The seven hills of Rome use their traditional English forms.
The apparently random choice of suffixes used make the native Italian adjectival forms of place-names are a complete guessing game, even for the Italians themselves. In general (outside the domains of cookery and football, see below) they should be avoided in English except where native English forms exist: Roman, Milanese, Florentine, Venetian, Genoese, Tuscan, Lombard, Sicilian, Sardinian, Ligurian, Piedmontese; note that (unlike Italian practice) these words are capitalized. Some obsolete English forms - parmesan, Latin, Tridentine - and native Italian forms - bolognese and calabrese - have gone off on their own to find new (largely culinary) meanings in English and should not be used in a general attributive sense for the places. Where there is no native English form, using the native Italian one comes across as either precious or pretentious; use alternative structures "people from Como" not "comaschi", "the architecture of Romagna" rather than "romagnuolo architecture".
The canonical form of the names by which Italian association football teams are generally known is the feminine of the adjective derived from the name of the town or city: Udinese, Jesina, Fiorentina, Reggina, Reggiana, and so on. Many other just use the name of the city - Roma, Siena, Torino, Bologna - or area - Lazio. Due to their origins amongst British migrants, a few have names which are the English forms of the place name: Genoa, Milan. It should be noted that the Italians universally refer to the two Milanese teams as "Inter" and "Milan" (spoken with the stress on the fisrt syllable) whereas the traditional anglophone usage is "AC Milan" and "Inter Milan" (although these forms are now slipping into disrepute except on ITV)
Cities in Slovenia and Croatia
At various times in its history Italy or the Italian states have occupied chunks of Istria (which has or had quite a large Italian-speaking population) and the Dalmatian coast. These names remain in current use and may be encountered on Adriatic ferry timetables and the like.
Other foreign placenames which may cause confusion
Subject to improvement without notice; please /msg with suggestions, queries, disputes, clarifications etc.