The Romans are NOT modern day Italians, since several barbaric tribes were enslaved or raged in and mixed their blood with Roman heritage. The name "Barbara", for example, became usual in the Roman empire, although it has had insulting connotations at first.
The Celts were also romanized to some extent. Look at English language: it's very akin to French. Roman ancestry is everywhere in Western world.
First, apologies to all because this is a threadjack and has jack squat to do with Civ5 confirmed details, however.... call me a history buff as I nitpick this stuff:
1) Modern Italians are descendants of many peoples, prominently including ancient residents of Italy (Latin peoples, as well as Greeks, Etruscans, etc.) and several groups that variously raided or settled in parts of Italy during the Medieval period, especially the Goths, Lombards, Arabs and (to a lesser extent) the Normans. Most Italian-Americans are from the part of Italy that had the heaviest Arab influence, FYI.
2) The English language is NOT descended from the Celtic language of the Ancient Britons (i.e. the people of Boudicca), that would be Welsh. The English language is NOT primarily a Latin-derived "Romance" language, like French. The English language is about 60 percent derived from Germanic roots in the Anglo-Saxon language of tribes that settled or conquered areas of Britain in the early Medieval era. The Anglo-Saxons spoke dialects that were close to those of the Frisians. The Anglo-Saxon language is also known as Old English and was the dominant language of England from about 500 A.D. to around 1100 A.D. By this time, it began to evolve through contact with the dialect of French spoken by the Normans, Scandinavian colonists living in northwestern France who overthrew the Anglo-Saxon kings in 1066 A.D. Norman French became the language of government while Anglo-Saxon remained the language of the people, but as time went on, they came together over the latter part of the Middle Ages. Modern English emerged around the 1500s with the spread of printing and widely distributed works like the King James Bible and Shakespeare's plays helped to standardize the language in spelling and grammar. So there are a lot of French-derived words in English. There are even some words derived directly from Latin origins, especially those used in the sciences. But English is Germannic at its core.
It's worth keeping in mind (and now I'm going to tie it back in to Civ5) that LANGUAGE, CULTURE and GENETICS do not follow the same lines. Genetic studies show that the average person who identifies as English might be culturally English, and speak a mostly Germannic language with heavy Roman/French influence, but genetically is likely to be closer to the Celtic peoples than the other groups. And when you try to apply these ideas to Americans, they make even less sense. Remember Civ3 where American citizens looked like Native Americans? Most major civilizations, which is what this game is, are melting pots to one degree or another, whether you are talking about Americans, Victoria's British Empire, Stalin's Russia, Cyrus' Persia, Suleiman's Ottoman Empire, Augustus' Rome, Alexander's Greece, Charlemagne's HRE, or Justinian's Byzantium. The marginal and questionable inclusions are the one's whose "empires" lacked ethnic, linguistic and cultural diversity. I'm not sure what the answer is, but the game designers need to think about these things carefully if they want to achieve the right balance of playability and realism. Historically, it's only been with the advent of Nationalism that cultural identity played as prominent a role in self-identification as the political accident of who ruled the place you lived. Perhaps they should consider thinking about "Nationalism" really means and having some sort of change in the way culture and other elements function once this idea begins to spread, somewhat similar to the way emancipation works in Civ4.