Here's where the Cajun idea came from:

'However, I was reading a mystery last week that was set in French Louisiana. A character used the phrase in the story.'

The medieval French idea is just a joke.

As an American, I never learned a lot about European languages. For example, I was just reading a novel where a Roman police inspector travels to Venice. A number of Venetians are talking in his vicinity, and he cannot understand them. I was surprised to discover that Italian can be that multifarious.

Last year I was on the island of Stromboli, and I took a picture of the map of Stromboli. The map has everything written in standard Italian (which I believe is the dialect of Rome) and in the dialect of Stromboli. They are noticeably different.

To sum up - there are a lot more languages out there than we think.