"Julien Breughel" is called "Jean Brueghel" in other reports of the film. I think that he must be Jan Brueghel the Younger. That real-life painter was the son of Jan Brueghel the Elder. He would have turned 15 in the year the film was set. The Duke mentions Jan's father's painting to Jan.

Jan lived in Antwerp at this time, I think. Boom seems to be a Flemish town close to Antwerp.

The Dutch (at least in Holland) were neutral and not invaded during the First World War. So erniemunger's comment about Dutch collaboration during that war seems wrong. Belgium was neutral until it was invaded. Perhaps the commenter meant "Flemings" when he said "Dutch" (I know nothing about any such collaboration: I'm not making an accusation).