Идеальный код требует нулевых комментариев. Это должно быть просто и понятно для начинающих.
Found the solution myself: It's necessary to create a .resx file for the sitemap in App_GlobalResources before one can access the resource keys.
IMO this is not 100% logical, but ResourceKeys are not supposed to be used for other things than localizing, once could argue.