gistfile1.rb

def set_locale
   locale = params[:locale].to_s

   if !locale.blank?
     cookies[:preferred_lang] = { :value => locale, :expires => 10.years.from_now }
     session[:update_lang] = locale  # remember preferred setting for this session
   else
     locale = cookies[:preferred_lang]
   end

   preferred = (locale.blank? ? request.user_preferred_languages : [locale]).map{|l| l.downcase.strip }
   
   best_match = preferred.find do |lang|
     AVAILABLE_LOCALES.include? lang                         # de-AT etc.
   end or preferred.map{|l| l[0..1] }.uniq.find do |lang|
     AVAILABLE_LOCALES.include? lang                         # en, de, etc.
   end or "en"                                               # fall back to en if unknown
   
   I18n.locale = best_match
end

routes.rb

scope "(:locale)", locale: /en|es|fr/ do
  # routes
end