What’s your browser language set to? Are you actually in Belgium?
If your browser is sending “Accept-Language: nl-BE, <other stuff>” I wouldn’t be surprised if they showed French ads. It’s one of the official languages of Belgium after all.
They may not have any relevant Dutch inventory to show, or they’re getting more money for the French-language ads which also target Belgium.
My browser language is English, and I live in Flanders. I never search, read or watch anything in French. So there is no reason to show me French ads, except that I live in a country where there is also a French speaking part.
I honestly don't care why they show French ads, but you cannot call such things "targeted", if you cannot even get the language correct, out of a choice of either Dutch or English.
If your browser is sending “Accept-Language: nl-BE, <other stuff>” I wouldn’t be surprised if they showed French ads. It’s one of the official languages of Belgium after all.
They may not have any relevant Dutch inventory to show, or they’re getting more money for the French-language ads which also target Belgium.