Other planets in the Nepleslian empire could *eventually* begin developing their own cultures. I know for a fact that I made Delsauria with a certain desert/Arabic culture in mind, and it was easy since it's been a part of the Nepleslian Empire for a while and was essentially a blank slate.
We have plenty of colonizable planets and systems right now, but most of them (especially in the Colonial Expanse) are pretty fresh. It takes time to develop a culture I'd wager, especially if you're already bringing in the stock Nepleslian culture to begin with.
That said, I don't see why certain areas of Nepleslia Core couldn't be culturally separate. Los Apagos is the SARP equivalent of a New York/Los Angelas type city, and we even have a China Town (Or Yammie Town, if you really want to get specific) so it wouldn't be too much of a stretch to have different customs or speech dialects on certain areas of the globe.