Does someone from Calais sound as different to someone from Marseilles as a Scot does to a Cornishman?

Well if the Northerner was speaking in full ch’ti-mi to another Northerner then I reckon most would struggle to understand.

He’ll yes. I used to have a house in TH SW of France, and the twangy accent is quite something.

As I understand it, the regional differences in accents are as obvious to the language speakers as ours are, particularly Germany.

So to a foreigner does a Brummy sound like a Cockerney?

Yes, my wife is from Provence and often had the mickey taken out of her accent when she was at university in Paris.
She sounded the equilvalent to someone from Cornwall.

It’s quite noticeable in the older generation.
“Domain” is pronounced more like Domeng in Provence, for example.