I wanted a picture of all of us with the castle behind us and asked a passer-by exiting the castle if they could please take our picture (in French). They did so and I thanked them. When they found out I spoke English they switched to it immediately and so did I. This is because they were from the north of Belgium and Flemish is their mother tongue.* They made it clear that they preferred English to French.
When they left, I reflexively said “merci beaucoup” without thinking and then, immediately apologized for having spoken in French. To make reparations, I attempted a “dank u”.
*I have always been advised when meeting Belgians who speak Flemish to speak English not French (one of their country’s languages and mandatory in school from an early age). I was told that they could feel slighted that I had chosen to learn one of their countries other languages before Flemish. I have always done so and found their English to be excellent.