i assume you downloaded the fr_FR.zip archive on the french forum, this archive is not yet upgraded to the C5.1 version, but it will be very soon.
in this archive you'll find a LISEZMOI text file that explain you how to structure your /languages folder to have the C5 interface in french - this is basically the translation of the README file + the translation of the second part of the Andrew's message:
http://www.concrete5.org/community/forums/internationalization/mult...
where he explains how your filesystem should look like and the basics modifications you'll need to do in the /config/site.php and how to have the install process in fench as well.

i reckon it's not the best place to donwload this archive, but keep in mind this is a work in progress, and this will be improved..

here are some threads that talk about multi-lingual site (there are maybe more):
http://www.concrete5.org/community/forums/chat/how_to_build_a_multi...
http://www.concrete5.org/community/forums/customizing_c5/page_struc...
i like the first approach Franz gave in the second link above

I like too the first approach of the second message link you gave. But it need something to copy all the webpages of one country to another new one, so it would be easier to translate after that.

Thanks.