Cannot translate the Mass Mailer plugin with Poedit


I tried to translate the Mass Mailer plugin using Poedit and the methodology you described in your article on plugin translation...

However, after uploading the updated language files it doesn't work.

Here's the exact process I followed :

1. Downloaded the "mass-mailer.pot" file on my hard drive. Opened it with PoEdit.
2. Set up the language to "fr_FR" (French translation)
3. Doing the translation within PoEdit.
4. Clicking on "Save".
5. Uploading "mass-mailer-fr_FR.pot", "mass-mailer-fr_FR.po" and "" to the languages folder of the plugin (...wp-content/plugins/mass-mailer/massmailerincludes/languages.
6. Hard refresh of the page of the plugin.

And nothing changes, the language displayed is still english.

Please note that yes, I made sure the language was set to "fr_FR" in the wp-config.php files, and I also tried different variations with the naming of the files.

What am I missing ? Thanks a lot in advance for your help !