Is it legal to announce results of an opponent's hidden card?
Because it is not explicitly mentioned in the rules  what you can or cannot talk about, there are two interpretations for this. I suggest that you agree with each other which interpretation you will follow before starting the game. (as this really can divide people's opinions)

Option 1: Do not reveal a card = do not even talk about the card

Interpret the verb "reveal" as "to make something known". Keep the hidden information hidden, and do not even talk about it. This is pretty simple rule, and it is the way-to-go, if any player disagrees with the other option. 

However, it also might be hard to define that how much information someone may give on someone elses (or his own) card: Is a surprised face too much? How about saying "Whoaa!"? What about "Whoaa! I should bow to You"? Or "I should bow to You, Sir/Miss"? 

Option 2: Do not reveal a card = do not show a card

Interpret the verb "reveal" as "to show". Now you can say anything about anyone's cards, but it is on the other player's decision that will they believe you or not. This gives people the opinion to bluff and make tactical moves e.g. when playing the Priest or the King.