Hi Fabien!
No, if you use the UTF-8 charset you need to look inside the lang/UTF-8/serendipity_lang_fr.inc.php file! There it's defined as UTF-8, not latin.
Yes, it's mandatory in French! (in French !)
Wow, I didn't know. Actually it looks like a mess to me, if I see spaces before "?" or "!".
It's more annoying for comments. Is there something to do?
Because of security issues, no HTML is ever allowed in comments, so the output will always be htmlspecialchar()ed.
I'm really sorry, I don't know a good solution to that. This is probably one of the reasons why the rest of the world does not put a space before punctiation characters

)
Best regards,
Garvin