I've witnessed a very interesting situation in a Wallonia supermarket, not far away from the border with Flanders. The Flemish guys in the supermarket were buying beer, and the Wallonian cashier had some problems with their credit card. They couldn't communicate with each other, as they didn't have a common language. Eventually, as my wife and I speak a bit of French, we asked her what the problem was, and then repeated it in English to the Flemish guys. It turned out that there was some issue with the card terminal not accepting Visa for some strange reason, and that she wanted them to try a different card. I always thought that the language barrier was overestimated, but it was quite plainly obvious that neither of them spoke the other language to a level that they could communicate.
What was surreal was that the till was only in French, and they had no way of switching it to Flemish so that they could show what the issue was.
I asked the guys where they were from, and they said that they were only visiting and that they were from near the Dutch border, so they had no contact with French on a daily basis, and that as the cashier didn't speak English, they had no common language. Afterwards, I asked the cashier out of idle curiosity about Flemish, and she said the the same story, that she simply didn't have much contact with Flemish and that it wasn't taught very well in school.