Ok. Found a way to make it work:
01. Put your site in Maintenance mode;
02. Visit your sub-site (multisite install): wp-admin/options-general.php
03. Go down to: Site Language
04. Swith to English or French
English will let Newsletter pull only the English versions of your contents
French will let Newsletter pull only the French versions of your contents
This is it. Not the most elegant way but still… It works!