I've been putting this one off for a while because it can get complicated. Not only were there noble titles in the 18th century, this everyone knows, but confusion arrives when (a) those titles are translated into French, (b) one person holds multiple titles, (c) the same title is applied to people of differing ranks, or (d) a title is held by someone nominally but without legal recognition, or (e) lots of people suddenly die (a la guillotine) or are forced to flee and the succession lines get very confusing.
Take for example the title "Prince". There are Princes of the Blood, i.e. those related to the Royal Family, like
Prince Louis Phillipe d'Orleans. To gain this title one would have to be descended from the male line of the reigning royal family, although a Princess of the Blood could marry into the title. Everyone remember the part in "Marie-Antoinette" where the Comtesse de Noilles is whispering about the proper greeting order, and who is a Princess of the Blood or only a Princess of the Blood by marriage? This is what she means.
What if someone is not "of the blood"? You can still be a Prince or Princess! How about the
Princess de Conde, Charlotte de Rohan? For her "Princess" was just a nominal title, something that the cadet branch of the Bourbon family was called, much as the distant descendants of royalty are still called today. They didn't rule a principality, were very far from being in line for the throne, and in her case was a Princess by marriage even. Interesting fact, if you look at her parentage Princess Charlotte's mother was Anne Marie Louise de la Tour d'Auvergne (familiar...) and the family's Turenne branch with it's Duchy of Bouillon were styled "princes of Turenne".
Not related in any way to the royal family? You can still be a Prince or Princess,
Prince Louis de Rohan, aka Cardinal Rohan of "The Affair of the Necklace" infamy, was! In certain very theocratic political systems, like France in the 18th century, very prestigious religious titles were given the honorific 'Prince', such as Cardinals and the Pope.
Obviously those who rule a principality are known as Princes, but where were these sovereign states in the 18th century? The Principality of Orange (as in William of Orange) was one, and so was the Principality of Sedan which was a part of the Duchy of Bouillon, and therefore became property of the French Crown when the Duchy was confiscated. Everything is related.
The title of Prince doesn't fall into the category of titles difficult to recognie in a foreign language, but here are some that people do mix up:-
Comtesse/Comte= Countess/Count
Marquis/Marquise= Marquess (Earl), Marchioness
Duc/Duchesse= Duke/Duchess
Roi/Reine= King/Queen
Cavalier/(Ma)Dame= Knight/Lady
So where do all of the Princes line up? Rank being important and all, who goes in what order? After the King would come the heir, known as the Dauphin, then his brothers, the Princes of the Blood, then the Princes of the Blood by Marriage, the Princes of Principalities, Princes styled by patent of the King, Princes styled hereditarily, and finally everybody else.
Then Napoleon came along and started to hand out Princely titles to his victorious generals, so things only became more confusing...