[master] Refactor i18n

[FranckKe]

Use browser language setting, fallback on english Remove unused backend i18n detection Remove default/fallback env i18n settings Update interface with user language on login Fix language switcher default value

To test:

1. Detect user browser language settings:
   • Disabled or remove application localstorage state
   • Set your browser language to es-* or fr-*
   • reload
   • Check if initialized language is browser language

     ---

2. Detect fallback if user browser language is not supported:
   • Disabled or remove application localstorage state
   • Set your browser language to anything else than en-* es-* fr-*
   • reload
   • Check if initialized language is english

     ---

3. Language updating on login
   • Login
   • Set language in settings
   • Logout
   • Set language in nav dropdown to a different one than the user setting
   • Login
   • Language should have been updated with the user language

[mdouchement]

Check /admin too. It might be broken after removing all the i18n in the ApplicationController