Dialects Can indeed do this. My aunt is british: Her mother's dialect was so thick I could hardly understand her as a child. Fast forward to adult life, and I moved to Norway. Spoken dialects here can have just a few hundred people, though writing is more standardized. To confound it all, there are two official forms of Norwegian. Norwegians are expected to understand not only both forms of the language, but most dialects... along with Swedish and Danish (Swedish sounds similar, danish is written similarly).

Yet... There are instances where two Norwegians simply cannot understand each other. They are more likely to switch to english or try very very hard to stick to plain language.

I have less luck with the dialects - I can understand some with a few alterations.

On the other hand, I now understand different dialects of English much better.