Oh no, another one of those "Why is MY civilization not in the game" threads
If the Celts, why not the Poles, who actually had an empire, and bequethed to the world Copernicus and Chopin
If the Celts, why not the Dutch, who once had an empire stretching from New York to Indonesia, invented the first stock exchange and central bank, and brought the world Van Gogh and Edam cheese.
If the Celts, why not the Italians, who gave us the Renaissance and Pizza.
If the Celts, why not the Portuguese, whose empire stretched from Brazil to Malaya.
If the Celts, why not the Turks, who ruled one of the world's most powerful empires, and gave us coffee and Turkish delight.
What's notable about the current line-up of civs is that most of the included civs were powerful and significant not just for a few centuries back in the Dark Ages but for a big chunk of history, including the Middle Ages and modern era. The understandable exceptions are the Greeks and Romans (who between them founded western civilization) and the Americans (who now dominate the world). Other civilizations which were big in ancient times but then eclipsed have not been included. Hence no Hittites, Babylonians, Assyrians, or Celts. IMHO this is quite correct. The Celts did not create a big empire. The Irish, Welsh. Bretons and Cornish have no doubt brought the world many wonderful things, but superpowers or even powers they ain't and weren't.