How to test Drop Feeds with a different language than you default language ?

1. Download and install the targeted language pack from addons.mozilla.org
2. Go on about:config page
3. Set intl.regional_prefs.use_os_locales to false
4. Set (or create a string value) intl.locale.requested to the the targeted language code, for example "fr"
5. Restart Firefox, now Firefox and Drop Feed work with the targeted language
6. To go back to your default language, go on about:config, then right click on intl.regional_prefs.use_os_locales then select "reset" and right click on intl.locale.requested then select "reset", and restart Firefox.