The first one that jumps to mind is the Western Roman Empire. At it's zenith no matter where you went in the Roman Empire, the language of business was Latin, and every citizen identified themselves as roman. At the end, in the west the provinces spoke their own languages and identified themselves by their ethnicity. They did not think of themselves as roman citizens and did not owe any fealty to that institution. That is one of the biggest reasons for it's ultimate fall. Heck Alan, it's happening now in your own backyard. The people of Quebec identify themselves more as citizens of Quebec than Canada. The more abstract the idea of Canada becomes to them, the closer they will move to secession. It would be a disaster for both groups. And language is one of the biggest wedge issues right?
The key word here is "citizen". Only a very small minority of Roman population were citizens. Also, Roman culture was mainly city based. Outside cities the only Romans you could find were small families living in villas.