I'm afraid I couldn't find it in English, but I did find this: http://gallica.bnf.fr/ark:/12148/btv1b8625375d
It's in (gasp) 17th century French, but I found it to be comprehensible using my French. Feel free to ask me if you need help though, French is one of my native languages.
18th century
Was it standard practice back then to refer to monarchs as a "Prince/Princesse" first and then by their official title?
The treaty speaks of "le Sérénissime & Très-Puissant Prince Louis XV, par la grâce de Dieu, Roy très-chrétien de France & de Navarre…" and uses the same type of formulation for the rest.
Maybe it's only French treaties that do that? Reading the Treaty of Paris,after the Seven Years' War, it looks like they use the term "Prince" again (the treaty was written in French). In the treaty of Hubertusberg between Prussia and Austria after the Seven Years War, both parties are referred to as "Konig", German for King.
Was it standard practice back then to refer to monarchs as a "Prince/Princesse" first and then by their official title?
The treaty speaks of "le Sérénissime & Très-Puissant Prince Louis XV, par la grâce de Dieu, Roy très-chrétien de France & de Navarre…" and uses the same type of formulation for the rest.
Not sure about this, but I think that "Princes" designated the ruling-class nobles, in the same way "Barons" was (slightly earlier) a generic term for all nobles. So Louis XV has (nature = prince, job = king).
Etymologically it fits: Princes < Lat. Princeps "first".
And it is quite usual, at least in French, in this time period. (For one obvious example, open Rousseau's Contrat Social where the Prince is the ruler).