Thank you for all the help Toscho! The issue was actually quite obvious after a night of sleep. I am using WP Native Dashboard. In case you don’t know what that is, it allows you to change the language of the Admin section of the site.
Considering WP Ajax is considered to be admin, it’s changing the language like so. Once I logged out, or changed the back end language to FR, it’s working as intended.
I’ll report this to the plugin author as a bug.