Zut alors, si ce n’était qu’une blague
Laugh-out-loud moment there! We just had an Australian house sitter who did exactly that - even to me. A bit like the Allo Allo policeman syndrome…
Something just not quite right in the head… 
The BBC article explained it by suggesting that some people might subconsciously adjust their speech and accent to match those around them - so if you are surrounded by people speaking English to you with a French accent you might start to pick up a French accent yourself. Bizarre.
The British habit of speaking English slowly and loudly to foreigners stems, surely, from the colonial notion that - of course - they speak English, but might not be very good at it so we have to make it easy for them.
It’s the sort of attitude which should have been buried with the last vestige of Empire but sadly clings on even today with some people.
We had a guest in our CdH just like this a few years ago, staying at the same time as a French friend who speaks English well. The gentleman’s wife had given up worrying about his antics long ago. She was only slightly embarrassed and totally used to it. He was a real character and he fearlessly and confidently spoke fluent English with a French accent. We couldn’t quite believe what was happening! Our friend found it as surprising and amusing as we did. The guy should have been on the stage.
Really hearing, listening to, the meaning in someone else’s speech, or whatever means they use, for communication, I think of as a precious/ valuable gift/skill/talent that is intrinsic in the work of finest counseling, and best counsellors and communicators, from Rogers to Rosenberg, and including god. Humans …including me, need not be so easily distracted by surface gloss or deformity. So much goes unheard, until its too late.
My husband’s French is coming along nicely, he’s more confident than me now.
But about 2 years ago he would not only speak broken French to French people but when resorting to English would leave out whole words from his own language. The French must have thought he couldn’t even speak Correct English. A very strange phenomenon which luckily has passed.
I know this video has been done to death but here it is again…