Here are the details regarding the applicable law in France. Good luck!
Most of the time, the person being photographed (or one of his/her possessions) will ask you more or less hysterically what you’re doing.
Er, a photo… 
It’s happened to me half a dozen times in France (almost never anywhere else), which is very few compared to the photos I’ve taken of strangers, either for a person being photographed, or for example when photographing a… factory (no no, not a nuclear power station, something that makes concrete, I think).
Like everyone else (I hope), I explained my project and, even though they didn’t really understand what, people left me alone.
Note that what you use to take the photo makes a huge difference. You’re not experiencing the same thing at all when you’re shooting with a smartphone or a compact, or even worse with a DSRL.
Abroad, on the rare occasion when someone came up to me because I was photographing him, I did what all foreigners usually do: “Sorry no understand.” (exaggerating my french accent, of course) while looking contrite. 