Well, it's also another example of her lack of actual perspective on the series and world-building. Jean-Claude and his maker using French because that's what they were is fine with me, actually. Another possible explanation is that you're talking about the period when the European council took its more or less final form. I can see the argument for that, actually.

I care less about that per se than about the fact that the French itself makes me wince, at this far remove from my having learned some. Get someone to vet it if you don't know the language.

But what's troubling in general in-universe is the fact that Hamilton maintains that no one in law enforcement or in the general public has any understanding of how to deal with vampires and weres, there's only bad anecdotal information out there, and someone with a B.S. in preternatural biology is the only expert out there. The only one. The best. If you can get a degree in it, there are other people out there who know the field. Does not compute.