Vous pouvez modifier la traduction comme ça :
\documentclass{yathesis}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[bpchem]{chemstyle}
\AtBeginDocument{%
\addto\captionsfrench{%
\renewcommand\listschemename{XYZ}
\renewcommand*{\schemename}{abc}%
\renewcommand\listschemename{XYZ}%
\renewcommand\schemename{abc}%
}%
\csname captions\languagename\endcsname
}
\begin{document}
\listofschemes
\ref{refdocument}
\begin{scheme}
\centering
\includegraphics{example-image}
\includegraphics{example-image-duck}
\caption{légende}
\label{refdocument}
\end{scheme}
\end{document}
