Starkiller was obvi huge, so what if people who grew up in different parts had their own society.

Like, sanitation is normally lumped with maintenance, so he should probably know what some things are but he's completely oblivious on what's what. Different regional words and customs would probably be a better reason on why Finn is so different and weird, better than just him being an ex-Stormtrooper. He's an ex janitor forced into combat and has no idea what's happening...