X-COM - New Canon

To be fair, the aliens were basically all slaves, iirc. The Elders conquered them one by one, Covenant-style, except they didn't allow any sort of free will at all. Any of their original cultures were likely fragmented horribly, or lost altogether; with little to nothing left, humanity was all they had for reference.
Granted, maybe it's unrealistic for them to have assimilated so easily (several decades might not be enough, let alone a few years), but City 31 is the epicenter for that sort of change so it might be a faster process there.

The setting is pretty much in the time between the war and now everything was either run down, ruled by a questionable band of mercenaries, or when their choice was a concentration camp or "play nice"

Seriously, the very first time human supremacists show their face, a faction of aliens are like "humans suck, we out"