Fix management of translation files

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 update doc (cf NXDOC-50) and Studio behaviour (cf NXS-1922, NXS-2017)

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)