Several posters have mentioned "Walloons" or "Wallonians" when meaning "Belgian French-speakers". That is quite wrong, in my experience. More than once I have found people from Wallonia to be well-mannered, soft spoken, and doing great efforts to learn some Dutch. And it does take a good deal of effort for them as their education system is rather poor, in this respect as in many. Whenever I observe manners I find unacceptable, it is from people from Brussels, rather than from Wallonia. There remains a certain mindset of "la capitale" (and its people) are superior to "la province". This shows in many ways, sticking to the language of la supérieure capitale is just one. (The same error occurred already in the 1968 manifestations that led to the split-up of the Leuven university: the main slogan was "Walen buiten" but the really unwanted people were rich bourgeoisie youngsters from Brussels, who did at the time display a manifest disdain for all others, Wallonians included. Some of them, at least, let us not generalise.)
For one example: I worked for several years in a bilingual state organisation. Our team included five Wallonians, two Flemish, and one guy from Brussels. There was exactly one person in the team who never spoke a single word of Dutch - now guess which one?
On a related note: a serious error, committed by many Flemish, is to switch to French whenever someone addresses them, in Dutch, but with difficulty, or even with some bit of an accent. This is well intended, but it is not a good idea. I have known several Wallonians who gave up trying to speak Dutch, in disgust, because those arrogant Flemish would not appreciate their efforts, not did they get any chance to practice, that way. Well can I understand them! My suggestion: when addressed in Dutch, by an obvious non-native speaker, reply in Dutch. Only do it with more than the usual care: avoid dialect words, articulate clearly, speak a bit slower. Like a pilot is supposed to to in R/T (to return to an aviation note
)