I haven't seen Chocolat - it's a film I have decided to avoid - but I don't see why actors speaking in their own voices would be a problem. In my experience, English-speaking actors using French accents tend to sound like Inspector Clouseau. It's far less distracting for them to use their own natural accents. I can't imagine what Enemy at the Gates would have been like if everyone had spoken with Russian accents. Entertaining probably.