The most likely answer is that the author,Tonino Benacquista, was unaware that the US Marshals are indeed capable of operating outside of the United States (they are, after all, the primary US law enforcement agency when it comes to hunting and tracking fugitives from justice) and simply wrote that the family's protectors while in hiding were an undisclosed federal law enforcement agency (e.g. 'The Feds")
Here is a line from the Allocine website about the film, which is titled Malvita in foreign release:
L'équipe n'a opéré que peu de changements sur l'histoire, modifiant légèrement les relations au sein de la famille Blake pour rendre les parents un peu plus émouvants, comme le remarque la productrice Virginie Besson-Silla. "Pour la première fois, je me suis contenté de suivre l’intrigue existante. J’ai simplement ajouté quelques dialogues pour que l’adaptation soit plus cinématographique. Mais, en toute honnêteté, la structure et les personnages étaient déjà en place", ajoute Besson.
Its rough translation is:
The team has made few changes to the story, slightly changing relationships within the Blake Family to make a little more emotional parents, as noted producer Virginie Besson-Silla. "For the first time, I was content to follow the existing plot. I just added some dialogues that adaptation is more cinematic. But honestly, structure and characters were already in place," Besson says.
This is from the director of the film Luc Besson.
So it seems that little beyond minor character changes occurred in the making of the film. The overall text and intent of the author seems to be intact.