Are the languages Walloon and French as well as Flemish and Dutch as closely related as American vs. British English?

Answer 1

I can't comment on the Walloon and French relation. As for Flemish and Dutch, the answer is yes.

When someone speaks Flemish to me, I have no problem at all understanding what is being said. To me, it sounds like Dutch, only more "musical'. It sounds nice. I understand from quite a few born and bred American speakers they have the same feelings regarding British English.