The contribution comes with support for 4 languages at this time: english, spanish, german and polish. If you want to have e-mails sent in french, you simply have to modify the language files: recover_cart_sales.php and stats_recover_cart_sales.php. You should find these files in admin/includes/languages/french folder. The e-mail text is in the first file. If, after applying modifications, you get a parse error, it means that you messed up with the ' or . "\n\n" . chars.
If you get through it, please post new language files to the forum, so that they can be included in the next releases of this contribution.