They do not speak napolitan in Bari. They speak Barese. It has some similarities to napolitan and partly descended from it but is quite different. Similar to the relationship between Sicilian and calabrese. Insist on calling a Barese guy napolitan, he will tell you off.
Oh for sure, there's definitely quite a bit of rivalry, but I always thought that the language they speak in Bari is called "Barese Neapolitan" which is a dialect variety. The Foggiano dialect is definitely even more similar to Neapolitan.
Whenever Napoli and Foggia football teams play each other it's sorta seen as a semi-derby.
