Voici deux options.
1- avec l'extension `translator`
\providetranslation{January}{Janvier}
\providetranslation{February}{Février}
\providetranslation{March}{Mars}
\providetranslation{April}{Avril}
\providetranslation{May}{Mai}
\providetranslation{June}{Juin}
\providetranslation{July}{Juillet}
\providetranslation{August}{Août}
\providetranslation{September}{Septembre}
\providetranslation{October}{Octobre}
\providetranslation{November}{Novembre}
\providetranslation{December}{Décembre}
----------
2- sans (ou avec) l'extension `translator`
\def\pgfcalendarmonthname#1{%
\translate{\ifcase#1\or \ifcase#1\or Janvier\or Février\or Mars\or Avril\or
Mai\or Juin\or Juillet\or Août\or Septembre\or Octobre\or
Novembre\or Décembre\fi}%
}
Décembre\fi}
----------
**Exemple complet**
\documentclass[french]{article}
\usepackage{lmodern}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{tikz}
\usepackage{translator}
\usetikzlibrary{calendar,folding}
\providetranslation{January}{Janvier}
\providetranslation{February}{Février}
\providetranslation{March}{Mars}
\providetranslation{April}{Avril}
\providetranslation{May}{Mai}
\providetranslation{June}{Juin}
\providetranslation{July}{Juillet}
\providetranslation{August}{Août}
\providetranslation{September}{Septembre}
\providetranslation{October}{Octobre}
\providetranslation{November}{Novembre}
\providetranslation{December}{Décembre}
\begin{document}
\sffamily\scriptsize
\begin{tikzpicture}[transform shape,
every calendar/.style={
at={(-8ex,4ex)},
week list,
month label above centered,
month text=\bfseries\textcolor{red}{\%mt} \%y0,
if={(Sunday) [black!50]}
}]
\tikzfoldingdodecahedron[
folding line length=2.5cm,
face 1={ \calendar [dates=\the\year-01-01 to \the\year-01-last];},
face 2={ \calendar [dates=\the\year-02-01 to \the\year-02-last];},
face 3={ \calendar [dates=\the\year-03-01 to \the\year-03-last];},
face 4={ \calendar [dates=\the\year-04-01 to \the\year-04-last];},
face 5={ \calendar [dates=\the\year-05-01 to \the\year-05-last];},
face 6={ \calendar [dates=\the\year-06-01 to \the\year-06-last];},
face 7={ \calendar [dates=\the\year-07-01 to \the\year-07-last];},
face 8={ \calendar [dates=\the\year-08-01 to \the\year-08-last];},
face 9={ \calendar [dates=\the\year-09-01 to \the\year-09-last];},
face 10={\calendar [dates=\the\year-10-01 to \the\year-10-last];},
face 11={\calendar [dates=\the\year-11-01 to \the\year-11-last];},
face 12={\calendar [dates=\the\year-12-01 to \the\year-12-last];}
];
\end{tikzpicture}
\end{document}
