(Moving this post on Princess Elisabeth's title from another thread.)
The official website and official statements use the style HRH Princess Elisabeth, Duchess of Brabant. This is the official
styling used by the Royal Court.
Princess Elisabeth, Duchess of Brabant | The Belgian Monarchy
Hare Koninklijke Hoogheid Prinses Elisabeth, Hertogin van Brabant, viert haar 18e verjaardag | De Belgische Monarchie
However, it is a traditional Belgian and Dutch practice for the public to unofficially
refer to the heir(ess) to the throne as Crown Prince(ss). Philippe was called Crown Prince Philippe in the Belgian media and Willem-Alexander was called Crown Prince Willem-Alexander in the Dutch media when they were the heirs to their respective thrones.
The term "Crown Prince or Crown Princess" was even used in the Royal Decree of November 12, 2015, referring to the position of heir to the throne.
Moniteur Belge - Belgisch Staatsblad
Based on what I see, this unofficial style normally is not used to refer to the spouse of the heir(ess). The media normally referred to Crown Prince Philippe and Princess Mathilde (rather than Crown Prince Philippe and Crown Princess Mathilde), and after Princess Elisabeth marries, my expectation is that the couple will be called Crown Princess Elisabeth and Prince X in the press, rather than Crown Princess Elisabeth and Crown Prince X.