\documentclass[french]{beamer}

\usepackage{babel}
\babelprovide[main, import]{french}

\begin{document}

\begin{frame}
\begin{definition}
D
\end{definition}
\begin{theorem}
T
\end{theorem}
\begin{corollary}
C
\end{corollary}
\begin{proof}
P
\end{proof}
\end{frame}

\end{document}