For the next five sessions, unrelenting? Because that's the scenario
@Hussar seemed to think was reasonable. Also, if you're going somewhere where you can't speak to the locals, how about a little prep?
Umm, any dungeon crawl? Remember, speaking "Undercommon" doesn't exist. Every group of goblins, orcs, and whatever speaks a different language. Heck, think about something like Keep on the Borderlands. There'S a dozen or so different tribes in the Caves, plus the Lizardfolk. There's pretty much zero chance the PC's will speak even a majority of the languages.
So, no, it's not an unreasonable scenario. Any travel scenario should feature not being able to speak the language. If your characters are from Waterdeep, you should not be able to speak to anyone in Neverwinter or Baldur's Gate. Cormyr? Nope.
When you start trying to be "realistic" about languages, that means that every couple of days travel you go, you encounter new languages.
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".