Heck, it wasn't that long ago that even a fairly geographically small area like England would have several different languages spoken, none of which can communicate with each other. French, English, Celtic, Latin and I'm sure there were more. Documents would be in French, Latin or English and, again, probably more. "Speak to the locals"? Locals means "someone who lives within about a hundred mile diameter circle". Anyone outside of that circle? New language time.
And that's with everyone being human. Imagine adding in a couple of dozen intelligent races as well. Why would elves all speak the same language? Or orcs? Or Giants? In a fairly small geographical area - say about a hundred mile across circle, you could easily have a dozen or more languages being spoken.
So, no, it's not unreasonable at all. But, again, we slam upside against, "I want my D&D to be realistic, but, only to the level of realism that I want".