\documentclass[a4paper,11pt,openany,french]{book}
\usepackage[french]{babel}
\begin{document}
\part{A}\part{B}\part{C}\part{D}\part{E}\part{F}\part{G}\part{H}\part{I}\part{J}\part{K}\part{L}\part{M}\part{N}\part{O}\part{P}\part{Q}\part{R}\part{S}\part{T}\part{U}\part{V}\part{W}\part{X}\part{Y}\part{Z}
\end{document}
After the 20th part (vingtième partie), the part number doesn't show up.



french.ldfonly contains a list of words for the first 20 parts. – leandriis Mar 04 '20 at 22:16