Policy Based Design - Conditional use of Base classes

In the hello world example of policy based design from wikipedia we have this very nice way of defining the implementation (policy-) specific member functions via using directives:

template <typename OutputPolicy, typename LanguagePolicy>
class HelloWorld : private OutputPolicy, private LanguagePolicy {
public:
// Behavior method.
void Run() const {
  // Two policy methods.
  Print(Message());
}

private:
using LanguagePolicy::Message;
using OutputPolicy::Print;
};

Now assume that you want to allow a user to forget implementing the Message method in the language policy. Hence, you define a fallback class

class DefaultLanguagePolicy {
protected:
  std::string Message() const { return ""; }
}

But how do you enable this default if Message is not implemented in the HelloWorld Class and discard it otherwise?

template <typename OutputPolicy, typename LanguagePolicy, typename DefaultLanguagePolicy>
class HelloWorld : private OutputPolicy, private LanguagePolicy, private DefaultLanguagePolicy {
public:
// Behavior method.
void Run() const {
  // Two policy methods.
  Print(Message());
}

private:
// what do I need to write to get this kind of behaviour:
if (LanguagePolicy has Message implemented) {
   using LanguagePolicy::Message;
} else {
   using DefaultLanguagePolicy::Message;
}
using OutputPolicy::Print;
};

Thanks for any suggestions that ideally allow to keep this nice using syntax.