We've mentioned languages before, but consider the common trope in multi-species settings that all elves speak "Elven", all dwarves speak "Dwarven", all Martians speak "Martian", all furry blue things from Alpha Centauri speak "Alpha Centauran". Now think about humans.

There may be some strong arguments to fragmenting your non-human races into different language and culture groups. The local dwarves speak Galzuk, while the dwarves over the mountain range speak Zikbal, and they can't understand one another. This means translators become important, nay, vital for things like travelling and trading and generally not accidentally getting into trouble. Another approach is to have mostly mutually intelligible dialects that slowly transmute as you cross larger tracts of geography, until eventually you reach a point where the local dialect is essentially a different language. All of these reflects reality.

This is not to say that languages need to be fragmented like this. A uniform culture enforced over a large area can impose a single language. Although this may become a common lingua franca for dealing with strangers, with people in various parts of the empire reverting to their local tongues for more insular communication. Languages in fiction can be fascinating and lead to all sorts of complications and adventures. Just ask Rawlinson and Bosworth Professor of Anglo-Saxon and later Merton Professor of English Language and Literature at Oxford University, J.R.R. Tolkien.

Transcript

Luke: So what are you going to do during the battle?
C-3PO: Didn't you look at the pilot profiles Red Two gave you?
Luke: Um...
C-3PO: Why do you think we need a command centre here?
Luke: Er...
C-3PO: The pilots are from a dozen different planets. They speak different languages!
Luke: I see...
C-3PO: I'll be translating! See how useful I am? See why you shouldn't melt me down?
Luke: Okay, you've proven useful for another day.
C-3PO: Have fun storming the Peace Moon!
Luke: Good bye, 3PO. Good work. Translate well. I'll most likely melt you down in the morning.
C-3PO: If any of us still have any atoms next to each other.