Quatre-vingt-dix vs. nonante
The French language has various words to express the number 'ninety', among which are 'quatre-vingt-dix' and 'nonante'. The choice of word depends on the regional dialects of French that are spoken in different parts of the world.
Quatre-vingt-dix
'Quatre-vingt-dix' is the standard way to say 'ninety' in the French used in France. It literally translates to 'four twenty ten', as the traditional French counting system is based on increments of twenty.
Il y a quatre-vingt-dix personnes dans la salle.
(There are ninety people in the room.)
Ma grand-mère a vécu jusqu'à l'âge de quatre-vingt-dix ans.
(My grandmother lived until the age of ninety.)
Nonante
'Nonante' is used to say 'ninety' in some parts of the French-speaking world, most notably Belgium, Switzerland, and some regions of Canada. It is part of a decimal counting system, as opposed to the vigesimal (based on 20) system used in France.
Il y a nonante personnes dans la salle.
(There are ninety people in the room.)
Ma grand-mère a vécu jusqu'à l'âge de nonante ans.
(My grandmother lived until she was ninety.)
Summary
'Quatre-vingt-dix' and 'nonante' both mean 'ninety' in French, but they are not interchangeable as they are used in different regions with differing counting systems. 'Quatre-vingt-dix', used primarily in France, follows the vigesimal system based on 20, whereas 'nonante', prevalent in Belgium, Switzerland and parts of Canada, belongs to a more straightforward decimal counting system.