Neapolis was rather unique compared to the other cities of Magna Graecia, in that she persisted in utilizing Greek on public monuments well into the 2nd century CE. Most other Greek cities in southern Italy had begun the switch to Latin exclusively by the middle of the 1st century CE, if not a century sooner. Normally, the pattern was a gradual switch to Latin in the public sphere, with a mixture of Greek, Latin, and other Italic languages in the private sphere (like graffiti, funerary inscriptions, etc).

For Neapolis, Greek inscriptions account for nearly 25% of the total corpus of surviving epigraphic evidence, and that includes a huge number of public inscriptions like civic decrees. It is very distinctive and it seems to suggest that the elites of Neapolis took extra care to retain a Greek character in their city, especially in its public business, and deliberately contrary to the assimilating forces happening elsewhere in Magna Graecia. Oddly, most of Neapolitan funerary evidence for the Roman period is in Latin, not Greek. I would not put much stock in that, however, as funerary evidence can often give a very skewed picture of a place (depending on what necropoleis have been excavated from what quarters, and when).

Lomas talks about this evidence and more in her great essay in A Companion to Roman Italy, ed. Cooley (2016).