AHC: Empire with two main linguistic groups as assimilated as Greeks and Romans in the Roman Empire

What real or fictional countries could have had two main ethnic groups separated by language and contiguous geographic areas, that identified extremely strongly as being the same, like Latin-speakers and Greek-speakers in the Roman Empire, yet still viewed everyone else as essentially "barbarians"?

As bonus challenge it cannot be a settler colony and could be formed after nationalism already exists in the late 18th-19th centuries.
 
An Umayyad conquest of Constantinople and the absorption of the Greek Imperial beuracratic structure into the Caliphate to fill in the gaps Islamic law had for governing such a large entity could get you two different court/educational linguistic cultures in a Greek speaking civil system drawn mainly from the Aegean and an Arabic religious system drawn from the Levant. Assuming a move of the court to Constantinople, then I could see such a duel identity develop ("To be in the House of Peace, you must know Arabic to be a proper Muslim and read the Quran, or know Greek to deal with the systems set up for non-Muslims")
 
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