Morons are everywhere. In Bruges and in Wallonia.
Not difficult to say 'Goeie dag - sorry ik spreek geen Vlaams' when you are Belgian.

Walloons like that justify - partly - why some Flemish reject us.

I don't understand why learning French to ask for bathrooms would help?

English is OK. Or as suggested some basic Dutch to be polite.

When asking for a bathroom DON'T do as one gal on our bike trips once did - she did not say anything in English but just waved her hands over her lower front, pointing that she needed to go!

Who would understand that.

Just know that WC is international for bathroom.