If I may be so bold...? While agreeing entirely, I would add the suggestion that one should avoid my own elementary error of some years ago. Soon after arriving in Brussels for the first time, I strode confidently to the bar and requested a beer in my best 20-years-since-schooldays French. While drinking the served-in-total-silence beverage, I took in the surroundings. Before ordering my next beer, in English, I apologised profusely to the barman for my ridiculous mistake and wretched inability to speak any Flemish. With that, the atmosphere improved dramatically.
Not fair to do it anywhere else in BE though, particularly given the historical dominance of the Walloons over the Flemish.
Apparently, in the WW1 Belgian Army, the Officer class were almost exclusively Walloon, and would give the day's orders in French to the men, finishing off with:
"Et pour les flamands, le mÃªme." (And for the Flemish, the same)