add site setting for setting locale from header

lib/validators/allow_user_locale_enabled_validator.rb

@@ -0,0 +1,18 @@
+class AllowUserLocaleEnabledValidator
+
+  def initialize(opts={})
+    @opts = opts
+  end
+
+  def valid_value?(val)
+    # only validate when enabling setting locale from headers
+    return true if val == "f"
+    # ensure that allow_user_locale is enabled
+    SiteSetting.allow_user_locale
+  end
+
+  def error_message
+    "You must first enable 'allow user locale' before enabling this setting."
+  end
+
+end