And of course there is this question, which recently popped up in Mandie's fan fiction story "Lizya & Georgie ~ Love, Power and Tragedy" in the Having Fun section:
NB the dramatis personæ are a young Russian grand duchess and a young Bavarian prince:
“Sie sprechen Deutsch, Herzogin (You speak German, Duchess)?” Georgie informally asked in his native tongue. (And a very stupid question too! - Tom added).
I understand that the "Duchess" in question is a Russian Grand Duchess? In that case she is
Großfürstin (Grand Princess) in German. I think that to a certain extent social equals can adress each other with titles in German as in Russian, but in German I have a feeling this is restricted to lower titles like
Gräfin, Countess, i.e. the ones where the titles usually is preferred to the more uncommon predicate. But I think it would be more usual to add a
Frau, or in the case of a young girl as here,
Fräulein: "
Sie sprechen Deutsch, Fräulein Großfürstin?" But I am not at all sure here - it is indeed a very interesting question: How did royals of equal age who didn't know each other intimately enough to use those well-known nicknames Ducky, Sandro, Vicky, Toria, Bertie, Nicky, Minny etc. really adress each other?