Improve Gettext compilation

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

config/dev.exs

@@ -58,6 +58,8 @@ config :logger, :console, format: "[$level] $message\n", level: :debug
 
 config :mobilizon, Mobilizon.Service.Geospatial, service: Mobilizon.Service.Geospatial.Nominatim
 
+config :mobilizon, Mobilizon.Web.Gettext, allowed_locales: ["fr", "en"]
+
 # Set a higher stacktrace during development. Avoid configuring such
 # in production as building large stacktraces may be expensive.
 config :phoenix, :stacktrace_depth, 20