You can then access your resources by using "GetLocalResourceObject(...)" for resources related to your current UserControl or Webpage/Form or "GetGlobalResourceObject(...)" for resources that are global to your application (App_GobalResources folder) and you'll automatically get the localized object in the current thread culture.
Often however you may have the case where you want to retrieve a resource object in a different culture than the current one. This can be achieve as follows (example for the german culture):
CultureInfo info = CultureInfo.CreateSpecificCulture("de-De");
string localizedString = Properties.Resources.ResourceManager.GetString("BackText", info);