"Highness" is utilized when talking about any member of a royal family, so in this context it works.Page 12. Pretty sure an Emperor would be referred to as "His Majesty". "Highness" is for princes, I believe.
Thanks for the TL.
Addressing to people of aristocratic rank and royalty can become very specific, at least in the British system. @dereth is actually correct; and the King, or Emperor is referred to as "His Majesty". Adjusted accordingly to "Her" if the ruler happens to be a Queen."Highness" is utilized when talking about any member of a royal family, so in this context it works.