part number (1, 2, 3) as word (one, deux, trois)?

part number (1, 2, 3) as word (one, deux, trois)?

Can someone please explain to me, why I get:
Part ONE
but then:
Part DEUX
Part TROIS



In addition to an explanation, any successful solution to this rather odd
phenomenon would be highly appreciated.



% !TEX TS-program = lualatexmk
\documentclass[oneside]{scrbook}
\usepackage[french,british]{babel}
\usepackage{fontspec}
\usepackage{relsize}
\usepackage{fmtcount} %For ''Part One'' instead of ''Part 1''
\usepackage{titlesec}
\renewcommand*{\partformat}{\partname~\numberstring{part}}
\begin{document}
\part{This is Just a Part}
\part{This is Yet another Part}
\part{And, in Fact, One More}
\end{document}