In 21 to 29 the t of vingt is sounded, 22, vint-deu, 23 vint troi etc. From 80 to 99 the t of vingt is everywhere mute, and the n is sounded nasal. Quatre-vingt-un is sounded quatre-vin-un.
Cent un is sounded san un; but 200 deu sanz hommes.
Mille, thousand has no plural form. deux mille 2000; deux milles means two miles. Mil is used in Christian era, l’an Mil huit cent, 1800.
In pronoun Tous, the is never silent and always pronounced like ç or ss.
In Belgium for 70, septante, for 80, octante, for 90 nonante are used.
“The Aural System” Supplement.