I just had the same issue and found a solution. The latest versions of Subscribe2 are missing the following lines (for localization matters):
//4tres - locale
$domain = 'subscribe2';
$locale = get_locale();
$mofile = ABSPATH . "wp-content/plugins/$domain-$locale.mo";
Place them at the top (not necessarily at the very top) of the plugins/subscribe2/subscribe2.php file
Then you have to place the 'mo' file within your plugins directory.
Because I had to write mine I downloaded a 'po' file in spanish (any language will do) from
Then installed poEdit from http://www.poedit.org/ , opened the subscribe2-es_ES.po, modified (translated) it, saved as subscribe-fr_FR.po, for I wanted to translate it into french. Make sure you saved both the 'po' and the compiled 'mo'. This last subscribe-fr_FR.mo is the file to place in your plugins directory.
Finally remember that it will take the language_country identification from your main wp-config.php file at the WordPress root folder.
Luck and have a good day,
hip - http://www.4tres.com