“oh, I know that language too!” from someone. And… They don’t.

Prime example: according to numerous surveys more Dutch people speak French well than Belgians.

Belgian is half French speaking, French is taught in Flemish schools from an early age, and many Dutch/Flemish speakers work in predominantly French speaking Brussels, and/or have French speaking friends/relatives.

But it's understandable: a Dutch person thinks their French is great if they manage to order something in a French restaurant while on holiday and have the waiter understand them. That's more than enough for their purposes. A Flemish Belgian thinks their French is bad, if they're unable to write a letter in French which respects rules like those about accord COD/COI (Elle a pris des photos. Les photos qu'elle a prises. Elle est allée prendre des photos. - Hope that's correct)

That and Dunning Kruger. If you have little competence in foreign languages, you don't know enough to know that you don't know enough.