Change \localename to \letterlocale

%

%    \begin{macrocode}

\NeedsTeXFormat{LaTeX2e}%

\ProvidesClass{muletter}[2018/12/05]%

\ProvidesClass{muletter}[2021/07/19]%

%    \end{macrocode}

%

% \tableofcontents

%    \end{macrocode}

% \end{macro}

% \end{macro}

% \begin{macro}{\localename}

% \begin{macro}{\letterlocale}

% \changes{2017/08/23}{2017/08/23}{\cs{locale} was changed to \cs{localename},

%   since the \textsf{babel} package reserves \cs{locale} now.}

% \changes{2021/07/19}{2021/07/19}{\cs{letterlocale} was changed to \cs{letterlocale},

%   since the \LaTeXe{} kernel reserves \cs{letterlocale} now.}

% \begin{macro}{\muletter@data@locale}

% |\localename|\marg{locale name} defines

% |\letterlocale|\marg{locale name} defines

% |\muletter@data@locale| as \meta{locale name}, where \meta{locale name}

% corresponds to the locale of a letter. The default value is

% either |\languagename| defined by the packages of \textsf{babel} and

% \textsf{polyglossia}, or |english|, when |\languagename| is undefined.

%    \begin{macrocode}

\muletter@def[locale]{localename}%

\localename{%

\muletter@def[locale]{letterlocale}%

\letterlocale{%

  \ifx\languagename\undefined

    english%

  \else