Allow locale to be set through the front-end

* Store the last used locale in the session
* Allow home page to be identified in helpers even when containing
  parameters
* Create initializer to explicitly set the available locales, the
  default and the load_path

app/controllers/application_controller.rb

@@ -4,7 +4,21 @@ class ApplicationController < ActionController::Base
   self.responder = ApplicationResponder
   respond_to :html
 
+  before_action :set_locale
+
   # Prevent CSRF attacks by raising an exception.
   # For APIs, you may want to use :null_session instead.
   protect_from_forgery with: :exception
+
+  private
+
+  def set_locale
+    if params[:locale] && I18n.available_locales.include?(params[:locale].to_sym)
+      session[:locale] = params[:locale]
+    end
+
+    session[:locale] ||= I18n.default_locale
+
+    I18n.locale = session[:locale]
+  end
 end