Distance and ease/frequency of contact would factor in, as well as the backstory.
A milieau where a single society colonizes multiple worlds over a short period of time and the colonies then maintained frequent communication with each other would be expected to *maintain* a common language, whereas less frequent contact would allow for drift and eventual differentiation, requiring a trade language instead of a universal language.
Conversely, a diaspora milieau, with different precursor societies would start with different languages and either evolve a common lingua franca if one did not exist but frequent contact required it or might make do with a trade language instead.
It's going to depend on the scenario the author chooses and how thorough their followthrough. And, of course; relevance. A lot of stories don't need to answer any questions about language if its not relevant to the story.
If you have Dick Seaton gallivanting across the cosmos meeting dozens of cultures, language issues will matter. If you have Kim Kinnison, with his lens, language won't matter a bit; only how many pirates ships he blows up.
