Usually the standard is implicit, but obvious from context, for instance if we were talking about racing, we could say that a Mclaren is better than a Volkwagen, and elide that we mean "better at moving quickly around a racetrack".
It certainly makes sense to judge cultures, as long as you remember to have a standard in mind. For instance, by the standard of the general wellbeing of African Americans, it's manifestly obvious that modern American culture is superior to American culture of the 1850s.
The fact that different cultures value different things shouldn't be a problem for discourse, as long as your interlocutors have values in common. Lack of objective truth about values shouldn't prevent one from working towards the values that they personally hold.
Relativism isn't (or at least shouldn't be) "there's no objective truth about values, so therefore every value system should be treated as equally valid". That would be an objective normative claim, and making it would be inconsistent with relativism. Relativism is just "there's no objective truth about values", and it stops there. Nothing about that prevents you from preferring your own values to those of others.