People learning Latin for modern history are very likely to read material referring to Bostonia or published Bostoniae. Latin isn't confined to Europe and wasn't confined to Europe even in ancient times (e.g. Sinae = China). I would like to see a wider range of place names to create opportunity to learn about them.
It's the locative case used for cities. Because Bostonia is first declension the "a" becomes "ae". Using the locative case is basically adding "in".
"Bostonia" means "Boston", "Bostoniae" means "in Boston", "Roma" means Rome, and "Romae" means "in Rome". It's also in the tips and notes.
I hear the B in Bostoniae, but the o sounds like an a. This must be because the speaker (female) is American, and American English widely lacks the short "o" sound and renders it as "a". Asking an American to say "o" properly is rather like asking an English person (or pretty much anybody else who isn't Welsh) ) to say the Ll in Llandudno.