LISA: Well, Ireland is the forefront of Europe’s tech boom.
BART: In your face, Belgium!
MARGE: Bart, if you hate Belgium so much, maybe I should take your Tintins away!
BART: No, I’ll be good!

There are many more mentions of Belgium in The Simpsons. The most exhaustive list I found are the 11 mentions in Belgium page in Wikisimpsons. The wiki also lists 3 appearances of Tintin and 3 mentions of Tintin (real or fictional) comics.

There is also an episode named “Les Schtroumpfsons” in Simpsons Comics #131 where Homer is door-to-door selling waffles.

Last but not least: