In Washington state, USA, however, we have only the slightest of differences between the rural eastern-side of the state and the urbanized western-side. And the urban accents are indistinguishable (at least to me) in a region that spans from Northern California all the way up to the Canadian border. Partially because of the Californian northward interstate migration in the 70's and 80's.
Which just suggests that there are models in the real world for either the "very hard to understand" obstacle (when needed for for your story) and for the "we all understand each other" trope (when you just want the story to move past this issue).
It also suggests that in more constantly mobile societies, not only do you understand the range of dialects and accents (in some cases the language) of your neighbors, but the accent/dialect of your people is likely to span a greater area where there are little changes.
Perhaps the rural communities of the Oslira valley have a range of accents/dialects, but the Dara Happa cities all have a strongly similar accent within one dialect. Perhaps that clan opposite of your neighbors is almost totally unintelligible to you, but then you never really met them either, since they stick mostly to their own lands.
I would suggest that it does not matter most of the time, but for completeness in describing the region, it is a factor in the way that has been described in previous messages to this thread.
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