FIX: Include resolved locale in anonymous cache key (#10289)

This only applies when set_locale_from_accept_language_header is enabled

lib/http_language_parser.rb

@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+module HttpLanguageParser
+  def self.parse(header)
+    # Rails I18n uses underscores between the locale and the region; the request
+    # headers use hyphens.
+    require 'http_accept_language' unless defined? HttpAcceptLanguage
+    available_locales = I18n.available_locales.map { |locale| locale.to_s.tr('_', '-') }
+    parser = HttpAcceptLanguage::Parser.new(header)
+    matched = parser.language_region_compatible_from(available_locales)&.tr('-', '_')
+    matched || SiteSetting.default_locale
+  end
+end