Answer: Language
Step-by-step explanation:
The divide between the Flemish Belgians and the French Belgians seems to stem from language. The Flemish have their own side of Belgium where street signs are in Flemish and the French Belgians have the same in their side of Belgium.
This situation where both languages are spoken has led to both cultures experiencing a divide with one side espousing the need to speak one language and the other side doing the same. If they spoke the same language, there would be no divide - all else being equal - because they would integrate more.