But, what if Diplomacy actually is is best ability?
I guess I just don't worry overmuch about this level of hair splitting. Granted, when we play, usually checks come out after some role play has been going on, so, it's not something I've ever seen that the player will start with "I diplomatize the queen".
But, I'm honestly not drawing a whole lot of distinction here. Whether his actions are couched in game language or not, it's pretty clear what the intent is, so, I roll with that.
I think none of us want to hear "I diplomatize the queen" or similar non-specific usage of skills. "I bluff the guard" is also lack-luster and non-specific.
I think the cue is, the statement is non-specific. "I Climb the Wall" uses the name of the skill, but it addresses the target of the skill and it's generally accepted what it means, including using handholds, the nearby trellis, etc. There is no nuance to it needed.
"I bluff the guard" or "I diplomatize the queen" tells me very little. I can guess that you intend to lie to the guard, but I have no idea of your approach or even objective. As a GM, what exactly am I supposed to tell you is the outcome, if I don't know what the intent was?
I think it is pretty fair that a player action regarding a skill needs to include the approach and goal/intent. "I bluff the guard to let me through by telling him I'm a messenger with an important letter from the Duke for the Duchess"
Whether your group requires speaking in character instead, they all should probably hold to that minimal standard. Or as GM, you should prompt for such info if hit with a "I bluff the guard" command.