title_body: How do I naturalistically merge two languages without a clear Substrate and Superstrate? 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?
upvoted_answer: In fact, there are a lot of possibilities, and many of them are attested in the history of natural languages.
  
  
  Majority language wins. This happened in China at least twice, it was conquered and ruled by foreign people (Mongolians, Manchu), but the ruling class became sinified. It also happened in medieval France and Spain, they were conquered and ruled be Germanic tribes (Franks and Burgundians for France, Vandals and Goths for Spain), but the majority language of the residents won out.
  Most prestigious language wins. This happend in antiquity in the Roman empire, and we see it in many post-colonial states in Africa. There are some prerequisites for that such as a working educational system helping that.
  Pidgins and Creoles emerge, and a Creole wins. This can be watched in Papua New Guinea, where Creole languages like Police Motu and Tok Pisin are becoming the de facto standard of communication.
  Something else happens. The case of English is not as clear as my points 1.–3. suggest: While the majority language finally won, it was heavily impacted and changed by the languages of the conquerors (Normans and Danes).
  EDIT Stable multiliguality. Quite rare in the wild, but Switzerland with a coexistence of Swiss German, French, and Italian is an example. The fourth language of Switzerland, Romansh, is under pressure, despite attempts of language preservation.