Excerpt taken from the book The Story of French (Ch. 7)
“Vous êtes Belge?” (“You’re Belgian?”) is a question Jean-Benoît is often asked in France, though more often in the south than in the north. He knows enough not to be flattered. For some reason the French love to laugh at Belgians. Belgian jokes are like Newfie jokes in Canada or Vermont jokes in New England (we can testify that the same cookie-cutter stories circulate freely between languages). But there is at least one legitimate reason why some French confuse Belgians and Quebeckers: Both produce diphthongs (combinations of two vowel sounds) for certain vowels and drag other vowels out in a way that Parisian French no longer does. The pronunciations of Belgians and Quebeckers are actually quite different, but years of language purism have dulled French ears to the nuances that distinguish Belgian and Quebec diphthongs. Typically, Belgians add an I after the sound É so that aller (to go) sounds like alleï. In words like bière (beer) they stretch the E (bee-ehr). Quebeckers typically stretch the E and the diphthong, which results in a pronunciation something like bee-ah-air. Belgians also tend to use the resources of French differently from French people, distinguishing between words that the French pronounce the same way, like brun (brown) and brin (twig) or bout (end) and boue (mud).
Belgians themselves disagree on what exactly constitutes the so-called Belgian accent. The strongest examples either come from native Flemish speakers who use French as a second language or citizens of Brussels, where the Flemish influence is by far the strongest. Elsewhere, native Belgians speak very normative French under the influence of Picard and Walloon, two dialects spoken on both sides of the Franco-Belgian border and therefore not specifically Belgian.