Sure but that's only useful for writing, and quickly becomes irelevant in speaking when you can't understand the dialect because every word is pronounced differently than what you know. Two languages can share the same grammar rules yet be completely different from one another becasue the words themselves are different.

>Especially at the border

That's a very silly argument. Firstly, border regions between countries often share dialect similarities because d'uh, geographical proximity. A virtual line in the dirt doesn't mean two people will suddenly speak vastly different when they've evolved together for centuries only separated by a few km.

And secondly, they're both native German speakers, of course they don't need translators between each others, but that wasn't my point. My point was specifically that foreigners who learn "vanilla" German might end up needing translators when they hear some of those Austrian dialects(maybe Bavarian too, IDK).