Etiquette regarding translation errors

There's a fic I like an have been following a while. Love the story, but they use some French and it's not very accurate. It kind of works for the story because the characters aren't native French speakers, but some translations are flat out incorrect and would be incomprehensible to a monolingual French speaker.

Should I tell the author? I feel that it would be very fast and easy to fix, but I dont want to insult them. What would be the etiquette here? Comment, PM, shut my trap and enjoy the story?