diff --git a/Gemfile b/Gemfile
index 3f5a46fb761..f0bcb313cb9 100644
--- a/Gemfile
+++ b/Gemfile
@@ -45,6 +45,8 @@ gem 'active_model_serializers', '~> 0.8.3'
 
 gem 'onebox'
 
+gem 'http_accept_language', '~>2.0.5'
+
 gem 'ember-rails'
 gem 'ember-source', '1.12.2'
 gem 'barber'
diff --git a/Gemfile.lock b/Gemfile.lock
index 2bc7adb15d5..d62fabd7e1a 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -127,6 +127,7 @@ GEM
     htmlentities (4.3.4)
     http-cookie (1.0.2)
       domain_name (~> 0.5)
+    http_accept_language (2.0.5)
     i18n (0.7.0)
     image_optim (0.20.2)
       exifr (~> 1.1, >= 1.1.3)
@@ -427,6 +428,7 @@ DEPENDENCIES
   highline
   hiredis
   htmlentities
+  http_accept_language (~> 2.0.5)
   image_optim (= 0.20.2)
   librarian (>= 0.0.25)
   listen (= 0.7.3)
diff --git a/app/controllers/application_controller.rb b/app/controllers/application_controller.rb
index 6d51b6a34f0..356610ff601 100644
--- a/app/controllers/application_controller.rb
+++ b/app/controllers/application_controller.rb
@@ -164,7 +164,15 @@ class ApplicationController < ActionController::Base
   end
 
   def set_locale
-    I18n.locale = current_user.try(:effective_locale) || SiteSetting.default_locale
+    if !current_user
+      if SiteSetting.allow_user_locale
+        I18n.locale = locale_from_header
+      else
+        I18n.locale = SiteSetting.default_locale
+      end
+    else
+      I18n.locale = current_user.effective_locale
+    end
     I18n.ensure_all_loaded!
   end
 
@@ -309,6 +317,18 @@ class ApplicationController < ActionController::Base
 
   private
 
+    def locale_from_header
+      begin
+        # Rails I18n uses underscores between the locale and the region; the request
+        # headers use hyphens.
+        available_locales = I18n.available_locales.map { |locale| locale.to_s.gsub(/_/, '-') }
+        http_accept_language.language_region_compatible_from(available_locales).gsub(/-/, '_')
+      rescue
+        # If Accept-Language headers are not set.
+        I18n.default_locale
+      end
+    end
+
     def preload_anonymous_data
       store_preloaded("site", Site.json_for(guardian))
       store_preloaded("siteSettings", SiteSetting.client_settings_json)
diff --git a/app/controllers/users_controller.rb b/app/controllers/users_controller.rb
index 3a9f77942f4..3a187f109d3 100644
--- a/app/controllers/users_controller.rb
+++ b/app/controllers/users_controller.rb
@@ -714,7 +714,12 @@ class UsersController < ApplicationController
 
     def user_params
       params.permit(:name, :email, :password, :username, :active)
-            .merge(ip_address: request.remote_ip, registration_ip_address: request.remote_ip)
+            .merge(ip_address: request.remote_ip, registration_ip_address: request.remote_ip,
+                   locale: user_locale)
+    end
+
+    def user_locale
+      I18n.locale
     end
 
     def fail_with(key)
diff --git a/app/helpers/application_helper.rb b/app/helpers/application_helper.rb
index d8b9634dcc3..4cc6834c5c8 100644
--- a/app/helpers/application_helper.rb
+++ b/app/helpers/application_helper.rb
@@ -60,7 +60,7 @@ module ApplicationHelper
   end
 
   def rtl_class
-    RTL.new(current_user).css_class
+    rtl? ? 'rtl' : ''
   end
 
   def escape_unicode(javascript)
@@ -111,7 +111,7 @@ module ApplicationHelper
   end
 
   def rtl?
-    ["ar", "fa_IR", "he"].include?(user_locale)
+    ["ar", "fa_IR", "he"].include? I18n.locale.to_s
   end
 
   def user_locale
diff --git a/spec/controllers/application_controller_spec.rb b/spec/controllers/application_controller_spec.rb
index 48a217039c8..49ddc8f8145 100644
--- a/spec/controllers/application_controller_spec.rb
+++ b/spec/controllers/application_controller_spec.rb
@@ -13,6 +13,10 @@ describe TopicsController do
     request.env['HTTP_REFERER'] = ref
   end
 
+  def set_accept_language(locale)
+    request.env['HTTP_ACCEPT_LANGUAGE'] = locale
+  end
+
   it "doesn't store an incoming link when there's no referer" do
     expect {
       get :show, id: topic.id
@@ -33,7 +37,7 @@ describe TopicsController do
       end
 
       it "uses the application layout even with an escaped fragment param" do
-        get :show, {'topic_id' => topic.id, 'slug' => topic.slug,  '_escaped_fragment_' => 'true'}
+        get :show, {'topic_id' => topic.id, 'slug' => topic.slug, '_escaped_fragment_' => 'true'}
         expect(response).to render_template(layout: 'application')
         assert_select "meta[name=fragment]", false, "it doesn't have the meta tag"
       end
@@ -51,7 +55,7 @@ describe TopicsController do
       end
 
       it "uses the crawler layout when there's an _escaped_fragment_ param" do
-        get :show, topic_id: topic.id, slug: topic.slug,  _escaped_fragment_: 'true'
+        get :show, topic_id: topic.id, slug: topic.slug, _escaped_fragment_: 'true'
         expect(response).to render_template(layout: 'crawler')
         assert_select "meta[name=fragment]", false, "it doesn't have the meta tag"
       end
@@ -114,25 +118,87 @@ describe TopicsController do
     end
   end
 
-  describe 'set_locale' do
-    it 'sets the one the user prefers' do
-      SiteSetting.stubs(:allow_user_locale).returns(true)
+  describe "set_locale" do
+    context "allow_user_locale disabled" do
+      context "accept-language header differs from default locale" do
+        before do
+          SiteSetting.stubs(:allow_user_locale).returns(false)
+          SiteSetting.stubs(:default_locale).returns("en")
+          set_accept_language("fr")
+        end
 
-      user = Fabricate(:user, locale: :fr)
-      log_in_user(user)
+        context "with an anonymous user" do
+          it "uses the default locale" do
+            get :show, {topic_id: topic.id}
 
-      get :show, {topic_id: topic.id}
+            expect(I18n.locale).to eq(:en)
+          end
+        end
 
-      expect(I18n.locale).to eq(:fr)
+        context "with a logged in user" do
+          it "it uses the default locale" do
+            user = Fabricate(:user, locale: :fr)
+            log_in_user(user)
+
+            get :show, {topic_id: topic.id}
+
+            expect(I18n.locale).to eq(:en)
+          end
+        end
+      end
     end
 
-    it 'is sets the default locale when the setting not enabled' do
-      user = Fabricate(:user, locale: :fr)
-      log_in_user(user)
+    context "allow_user_locale enabled" do
+      context "accept-language header differs from default locale" do
+        before do
+          SiteSetting.stubs(:allow_user_locale).returns(true)
+          SiteSetting.stubs(:default_locale).returns("en")
+          set_accept_language("fr")
+        end
 
-      get :show, {topic_id: topic.id}
+        context "with an anonymous user" do
+          it "uses the locale from the headers" do
+            get :show, {topic_id: topic.id}
 
-      expect(I18n.locale).to eq(:en)
+            expect(I18n.locale).to eq(:fr)
+          end
+        end
+
+        context "with a logged in user" do
+          it "uses the user's preferred locale" do
+            user = Fabricate(:user, locale: :fr)
+            log_in_user(user)
+
+            get :show, {topic_id: topic.id}
+
+            expect(I18n.locale).to eq(:fr)
+          end
+        end
+      end
+
+      context "the preferred locale includes a region" do
+        it "returns the locale and region separated by an underscore" do
+          SiteSetting.stubs(:allow_user_locale).returns(true)
+          SiteSetting.stubs(:default_locale).returns("en")
+          set_accept_language("zh-CN")
+
+          get :show, {topic_id: topic.id}
+
+          expect(I18n.locale).to eq(:zh_CN)
+        end
+      end
+
+      context 'accept-language header is not set' do
+        it 'uses the site default locale' do
+          SiteSetting.stubs(:allow_user_locale).returns(true)
+          SiteSetting.stubs(:default_locale).returns('en')
+          set_accept_language('')
+
+          get :show, {topic_id: topic.id}
+
+          expect(I18n.locale).to eq(:en)
+        end
+      end
     end
   end
 
diff --git a/spec/controllers/users_controller_spec.rb b/spec/controllers/users_controller_spec.rb
index eb83ce115f5..d4d4965efbf 100644
--- a/spec/controllers/users_controller_spec.rb
+++ b/spec/controllers/users_controller_spec.rb
@@ -479,6 +479,15 @@ describe UsersController do
         email: @user.email
     end
 
+    context 'when creating a user' do
+      it 'sets the user locale to I18n.locale' do
+        SiteSetting.stubs(:default_locale).returns('en')
+        I18n.stubs(:locale).returns(:fr)
+        post_user
+        expect(User.find_by(username: @user.username).locale).to eq('fr')
+      end
+    end
+
     context 'when creating a non active user (unconfirmed email)' do
 
       it 'returns a 500 when local logins are disabled' do
@@ -1181,6 +1190,19 @@ describe UsersController do
 
         end
 
+        context 'a locale is chosen that differs from I18n.locale' do
+          it "updates the user's locale" do
+            I18n.stubs(:locale).returns('fr')
+
+            put :update,
+                username: user.username,
+                locale: :fa_IR
+
+            expect(User.find_by(username: user.username).locale).to eq('fa_IR')
+          end
+
+        end
+
         context "with user fields" do
           context "an editable field" do
             let!(:user_field) { Fabricate(:user_field) }
diff --git a/spec/helpers/application_helper_spec.rb b/spec/helpers/application_helper_spec.rb
index fb97b0454cc..5931ec0d3a6 100644
--- a/spec/helpers/application_helper_spec.rb
+++ b/spec/helpers/application_helper_spec.rb
@@ -84,5 +84,16 @@ describe ApplicationHelper do
     end
   end
 
+  describe '#rtl_class' do
+    it "returns 'rtl' when the I18n.locale is rtl" do
+      I18n.stubs(:locale).returns(:he)
+      expect(helper.rtl_class).to eq('rtl')
+    end
+
+    it 'returns an empty string when the I18n.locale is not rtl' do
+      I18n.stubs(:locale).returns(:zh_TW)
+      expect(helper.rtl_class).to eq('')
+    end
+  end
 
 end