I have to admit French people are very very bad for learning new languages and our English is not so good at school - compared with Scandinavian or German students - but nobody here would have such a bad accent with such a good and sophisticated vocabulary. Like.. she could write fluently without looking for her words but her oral is just so exaggerated.
I'm not saying she should waffle but as a French speaker and English learner, that's just so funny.
Well, I used to work for an Italian material provider, where the big bosses usually came shipping in from time to time and some even worked here. These where highly educated people, and they even wrote the standards and everything in English as it was a very multinational company. Anyways, they proclaimed that they did not have the "Es" as you hear in super Mario and the likes, you know the stereotypical Italian accent. But truth be told, they where the only ones who did not hear it. They really had that stereotypical accent.
I too used to get ashamed at our prime ministers lousy English, but truth be told even if I myself can imitate some rather decent oxford English, at some point I just don't bother. I feel it takes away focus from the task at hand and the identity in the accent communicate an extra element. If you stick to real British people tend to forget who they are dealing with. I guess this is also why Scotts, Irish and the likes (US perhaps) has their own traits as well.
As for my experience with the French in particular, I've been to Paris a couple of times, 4-6 days at the time. The experience I've had is that it's only the tour guides that speak British without an accent, and I don't even know if they really was French. Also I got the impression that the people of Paris is very proud of their language and cultural heritage, and if forced to speak English (which many dislike) they will keep the dialect as an added identity. I do not know if this is an universal norm for all of France however, and it might just been a statistically coincidence, where the reality is very different from my impression.
Then again it might be a generational thing. We in Scandinavia used to be the worst before we became among the better, where people over a certain age take a serious bump in the quality specter (or might not even know it at all). So ye, I guess it is a lot of elements to consider.
Never the less I really felt Helena fit with the impression I had of how French approach English, but again when it comes to something so subjective it is difficult to underline a correct answer where there is so many variables to consider.
Here you have our own Kayane in Emperor Cow's tutorial videos for some reference:
Just to take it back to point, I REALLY hope that they add a lot of Swedish/Scandinavian in Marie's accent. That would add towards the character's identity.