1 files changed, 30 insertions, 0 deletions
diff --git a/app/controllers/concerns/preferred_language_switcher.rb b/app/controllers/concerns/preferred_language_switcher.rb
index 872652100c9..5ac75e40b04 100644
--- a/app/controllers/concerns/preferred_language_switcher.rb
+++ b/app/controllers/concerns/preferred_language_switcher.rb
@@ -2,6 +2,8 @@
 
 module PreferredLanguageSwitcher
   extend ActiveSupport::Concern
+  include Gitlab::Utils::StrongMemoize
+  include PreferredLanguageSwitcherHelper
 
   private
 
@@ -11,8 +13,36 @@ module PreferredLanguageSwitcher
 
   def preferred_language
     cookies[:preferred_language].presence_in(Gitlab::I18n.available_locales) ||
+      selectable_language(marketing_site_language) ||
+      selectable_language(browser_languages) ||
       Gitlab::CurrentSettings.default_preferred_language
   end
+
+  def selectable_language(language_options)
+    language_options.find { |lan| ordered_selectable_locales_codes.include?(lan) }
+  end
+
+  def browser_languages
+    http_language_header = request.env['HTTP_ACCEPT_LANGUAGE']
+    return [] unless http_language_header
+
+    http_language_header.tr!('-', '_').split(%r{[;,]}).reject { |str| str.start_with?('q') }
+  end
+  strong_memoize_attr :browser_languages
+
+  def marketing_site_language
+    return [] unless params[:glm_source]
+
+    locale = params[:glm_source].scan(%r{(\w{2})-(\w{2})/}).flatten
+
+    return [] if locale.empty?
+
+    [locale[0], "#{locale[0]}_#{locale[1]}"]
+  end
+
+  def ordered_selectable_locales_codes
+    ordered_selectable_locales.map { |locale| locale[:value] } # rubocop:disable Rails/Pluck
+  end
 end
 
 PreferredLanguageSwitcher.prepend_mod