For my latest worldbuilding project I'm trying to create a naturalistic language for a fictional empire I've been creating. This empire was born at the end of the Bronze Age when various kingdoms and city states voluntarily united to defend against the growing power of a thalassocracy.
Though the story of the people who speak this language is more or less ironed out, I can't figure out how to proceed when devising a language for this multiethnic, multilingual and not-very centralised empire.
Though the languages spoken within the empire would all be related sister-languages derived from the same Proto-Lang I don't know how to determine wether one of these languages becomes more "prestigious" while the others remain as dialects.
Do I just pick the language with more speakers? Do I just pick the language that the Emperor speaks, even if the rest of the (very powerful) nobility might speak another language entirely? Do I unite the languages in a single koinè? If so, how?