This would be very useful to us. If a post is not already translated into user's language, there would be a button to translate that post (instead of the entire page like the Wordpress translator plugin)

yes.
it's a good idea.

will you use automatic translation or something to realize a multilanguage blog (in wich we edit the post in different languages)?

I prefer the second option to allow the readers to switch from a language to another.

http://forums.b2evolution.net//viewtopic.php?t=11158
another developer will need to do this request.

People still think this a good idea? (Well do people NEED this?) I will get onto it sometime soon if so.