Since changes to "fr_FR" format, mechanism is not consistent everywhere: addons still contribute to fr while fr is copied/initialized from "fr_FR".
Behaviour is not the same for "en_US" (not copied to en)
=> need to have a consistent behaviour:
- do not maintain a "messages.properties" file as fallback if not needed (or make its content the same than messages_en.properties)
- copy en_US messages to both en and en_US
- copy fr_FR messages to both fr and fr_FR
- in platform-lang as well as addons contributing their own translation files
Also default translations are not consistent (not the same items in en and "fr_FR" in platform-lang -> it would be fairly easy to define a test that checks for inconsistencies)