Introduce I18nConfig as single point of truth

It will be used in application.rb and views.
See https://github.com/openfoodfoundation/openfoodnetwork/issues/2113

lib/open_food_network/i18n_config.rb

@@ -0,0 +1,22 @@
+module OpenFoodNetwork
+  # Provides access to the language settings.
+  # Currently, language settings are read from the environment.
+  # See: config/application.yml
+  class I18nConfig
+    def self.selectable_locales
+      ENV["AVAILABLE_LOCALES"].andand.split(/[\s,]/).andand.map(&:strip) || []
+    end
+
+    def self.available_locales
+      (selectable_locales + [default_locale, 'en']).uniq
+    end
+
+    def self.default_locale
+      ENV["LOCALE"] || ENV["I18N_LOCALE"] || source_locale
+    end
+
+    def self.source_locale
+      "en"
+    end
+  end
+end