Formal:
* Gelukkige verjaardag! (This literally translates to "Happy birthday," but it's used for anniversaries as well.)
* Proficiat met jullie huwelijksverjaardag! (This means "Congratulations on your wedding anniversary.")
* Van harte gefeliciteerd met jullie huwelijksverjaardag! (This means "Heartiest congratulations on your wedding anniversary.")
Informal:
* Gefeliciteerd met jullie verjaardag! (This means "Congratulations on your anniversary.")
* Proficiat met jullie jubileum! (This means "Congratulations on your anniversary.")
More romantic:
* Veel geluk met jullie huwelijksverjaardag! (This means "Good luck on your wedding anniversary.")
* Proficiat met jullie liefde! (This means "Congratulations on your love.")
Remember that Flemish is very close to Dutch, so these phrases are also understood in Dutch-speaking areas.