I was just in France last week suffering through the same problem. I’m American, so I want to keep my system set to English both due to familiarity of the interface and because I want data in familiar units (miles, degrees Fahrenheit), but it kills me when every street and place name is absolutely butchered.
An interesting observation I made last week was that the poor synthesis of French street names in English mode was worse than American-mispronouncing-French level. I wonder if that exposes some level of mismatch between how the synthesis engine models language and how humans do.
In the case of French names it probably depends on if you have any exposure to French pronunciation rules at all. I have a pretty crappy level of high school French but I can usually manage not to butcher pronunciation. I was with a friend in Montreal though and she had absolutely no idea how to pronounce many things.
