let's do something clever and actually parse the accept lang header

app/controllers/application_controller.rb

@@ -8,12 +8,20 @@ class ApplicationController < ActionController::Base
   before_action :set_locale
 
   def valid_l(l)
-    return l if l && I18n.available_locales.include?(l)
+    return l if l && I18n.available_locales.include?(l.to_sym)
     nil
   end
 
   def extract_locale
-    (request.env['HTTP_ACCEPT_LANGUAGE']||'').scan(/^[a-z]{2}/).select{|l| valid_l(l)}.first
+    req_langs = request.env['HTTP_ACCEPT_LANGUAGE'].to_s.split(",").map do |lang|
+      l, q = lang.split(";q=")
+      [l, (q || '1').to_f]
+    end
+    return nil if req_langs.empty?
+    req_langs = req_langs.sort_by(&:last).map(&:first).reverse
+    req_langs = req_langs.map{|l| l[0..1]}.uniq
+    available = req_langs.select{|l| valid_l(l)}
+    available.first unless available.empty?
   end
 
   def set_locale