[Fix #1389] Force French language

In #1383, a misconfigured gem was corrupting a global I18n variable
that was persisten across requests, resulting in #1389, #1392 and #1398.

This commit prevents future corruptions of locales.

config/application.rb

@@ -20,6 +20,7 @@ module TPS
     # config.i18n.load_path += Dir[Rails.root.join('my', 'locales', '*.{rb,yml}').to_s]
     config.i18n.default_locale = :fr
     config.i18n.load_path += Dir[Rails.root.join('config', 'locales', '**', '*.{rb,yml}')]
+    config.i18n.available_locales = [:fr]
 
     config.autoload_paths += %W(#{config.root}/lib #{config.root}/app/validators #{config.root}/app/facades)
     config.assets.paths << Rails.root.join('app', 'assets', 'javascript')