In Switzerland during WWII, the population was advised to be suspicious of anyone not speaking the local dialect, especially of they asked for directions. The army removed all traffic signs in case of an invasion [0] and stores did not sell road maps anymore. Swiss German is (as most dialects) notoriously difficult to learn in later life for speakers of other dialects.

(Slightly related story: some years ago, I traveled back to Germany by train. A few minutes past the border, German border control went through the train and asked me for my ID card. I then realized that I had left my German ID card in the scanner at home a few days earlier because I needed a copy of it. The border control then naturally went through my backpack, which (what I then realized with horror) contained ca. 30 packs of matches. A few weeks earlier, we had a BBQ at a local lake and realized on the way that we had no matches to lit the fire, so we went to the next supermarket and just bought the cheapest offer, which happened to be a pack of 30. So there I was, crossing the border with no identification and nothing on me but a backpack full of matches. When they asked why I had so many matches on me, my 20-year old brain went into full self-destruction mode and I stuttered the following: "He-he-he, its not enough to build a bomb, isn't it?".

Luckily, I have an accent which makes it absolutely clear that I grew up around a major city in my home state. After asking me some questions about my birth town, other personal facts and making some calls, they didn't even fine me. I am quite sure that this would've been slightly more complicated if I had spoken perfect standard German, or had a foreign accent.)

[0] https://ulis-buecherecke.ch/pdf_infos_zur_schweiz/die_schwei...