I'm trying to find the best translation for the Russian term Сиятельство. (I can't speak Russian either! So don't expect too much from me in that regard.) This is a form of address, often translated into English as Illustrious Highness. In the old Russian Empire, it was the default form of address for princes and princesses not part of the imperial family. (With the tsar's blessing, some princely families used the higher style of Serene Highness, but let's not get into all that!)
The literal translation of Her Illustrious Highness would, I think, be Son altesse illustre. However, I know from the Monégasque princely family that their style - the official English form of which is His/Her Serene Highness - is Son altesse sérénissime, and not Son altesse sereine. Might Son altesse illustrissime then be a more elegant translation? Or is that just nonsense?