...OTOH, I could easily see laws being written against the use of titles of nobility, like in modern Germany.
There's no such law in Germany. All german noble titles were abolished and the former titles are now part of the last name, i.e. Ernst August
Prinz von Hannover (NOT
Prinz Ernst August
von Hannover), the husband of Princess Caroline of Monaco. That guy has no noble title, although not even most germans realize that.
But as a german citizen you can still use a foreign noble title, there are no laws against it.
BTW, Caroline is styled in other countries "Her Royal Highness The Princess of Hanover", although this title does not exist, she should be styled "Her Serene Highness the Hereditary Princess of Monaco" instead.