I still think I’d look at the DM with a sense of “the fuq?”If it literally was not a thing that existed in the world, then the PC obviously cannot refer to it either. So In such a situation the GM should inform the player when they try to issue the command, so that they can choose something else.
Though I think "formal greeting" is a concept that has existed for a very long time, so the idea that it would not be a thing seems rather strange to me.
It’s a gotcha moment.