fix locale selection, when locale is not available

app/controllers/application_controller.rb

@@ -7,12 +7,17 @@ class ApplicationController < ActionController::Base
 
   before_action :set_locale
 
+  def valid_l(l)
+    return l if l && I18n.available_locales.include?(l)
+    nil
+  end
+
   def extract_locale
-    (request.env['HTTP_ACCEPT_LANGUAGE']||'').scan(/^[a-z]{2}/).first
+    (request.env['HTTP_ACCEPT_LANGUAGE']||'').scan(/^[a-z]{2}/).select{|l| valid_l(l)}.first
   end
 
   def set_locale
-    I18n.locale = params[:l] || session[:locale] || extract_locale || I18n.default_locale
+    I18n.locale = valid_l(params[:l]) || valid_l(session[:locale]) || extract_locale || I18n.default_locale
     session[:locale] = I18n.locale
   end