I installed the Language Package based on this thread that you propose : http://fr.scribd.com/doc/82768844/Joomla-2-5-multi-language-website-without-using-3rd-extensions-in-10-steps
Unfortunately, all my links now bring me to Error 404. I presume it's only a redirection matter or something like this. (as suggested in the thread I tried redirecting dead links to the home page but it seems that it broke something...).
Actually I tried reaching my url with http://www.managium.ca/index.php/fr or /eng and it doesn't work. I also try links that were previously working but are not available anymore!
I can provide you with access to my backend if that could help because I don't know where to start at all...
thank you for your help!