hyperref warning caused by Beamer appendix

You can work around this warning by using

\documentclass{beamer}
\title{The Title}
\author{The Author}


\renewcommand\appendixname{Appendix}

\begin{document}

\begin{frame}
\titlepage
\end{frame}

\section{First Section} 
\begin{frame}\frametitle{First Section} 
This is a slide.
\end{frame}

\appendix

\section{Appendix}
\begin{frame}\frametitle{Supplemental Material} 
This is a supplemental slide.
\end{frame}

\end{document}

EDIT

The Reason for the warning was explained in @Gonzalo Medina's comment

The problem is the definition of \appendix in beamerbasesection.sty which uses \part{\appendixname} and \appendixname is defined in beamerbasemisc.sty as \translate{Appendix}

Related issue on github:

https://github.com/josephwright/beamer/issues/449

Macro \translate is defined in package translator. It is defined with an optional argument with many assignments in the implementation. Therefore it is not expandable and cannot be used in bookmarks this way.

The following code provides \bkmtranslate, a poor man's definition of \translate without support for an optional argument. Also the language search of the original \translate is not implemented. Instead, the language is defined as macro \bkmtranslateto. The default is \languagename. If the translation is not found, then the argument is used without modification, the same behavior as the original \translate except, that the original \translate can extend the search to more than one language.

\pdfstringdefDisableCommands is used to install \bkmtranslate as replacement for \translate in the bookmarks:

\documentclass{beamer}

\makeatletter
\newcommand*{\bkmtranslateto}{\languagename}
\newcommand*{\bkmtranslate}[1]{%
  \ifcsname tr@@@\bkmtranslateto @#1\endcsname
    \csname tr@@@\bkmtranslateto @#1\endcsname
  \else
    #1%
  \fi
}
\pdfstringdefDisableCommands{\let\translate\bkmtranslate}
\makeatother

\usepackage{bookmark}% not needed here, but faster bookmark updates

Using the babel package also resolves the issue as \translate{Appendix} produces a decent name.