The showlabels package results in a compilation failure when a Hebrew document is rendered with polyglossia+xelatex

Question

The following LaTeX code is saved in the file ~\test.tex:

\documentclass{article}
\usepackage{amsthm}
\newtheorem{theorem}{Theorem}
\usepackage{polyglossia}
%\setmainlanguage{hebrew}
%\setmainfont{Arial}
\usepackage{showlabels}
\begin{document}
\begin{theorem}\label{mythm}
A house is not a home.
\end{theorem}
\end{document}

When the following commands are executed in the Terminal:

cd ~
xelatex test

a PDF file is generated at the path ~\test.pdf. When opened in a PDF viewer, the file displays as follows:

If now the two commented lines are uncommented, and the xelatex test command is rerun, the execution fails, and the ~\test.log file contains the following snippet:

! Package polyglossia Error: The current latin font Arial(0) does not contain t
he "Hebrew" script!
(polyglossia)                Please define \hebrewfont with \newfontfamily comm
and.
See the polyglossia package documentation for explanation.
Type  H <return>  for immediate help.
 ...
l.15 \begin{theorem}\label{mythm}
? 
! Emergency stop.
 ...
l.15 \begin{theorem}\label{mythm}
End of file on the terminal!

However, if now the showlabels line is commented, and the xelatex test command is rerun, the execution succeeds, and a new ~/test.pdf file is created. When opened in a PDF viewer, the file displays as follows:

(Note that Hebrew is a right-to-left language.)

Why does the second execution fail? How can the problem be fixed?

Answer 1

The showkey-label is printed in typewriter and polyglossia errors as the default typewriter font can't handle hebrew. But polyglossia's error message are rather bad here, as the proposed fix doesn't work.

Either use for all three font families a font that can handle hebrew, or define a suitable hebrew fallback (some mix works too):

\documentclass{article}
\usepackage{polyglossia}
\setmainlanguage{hebrew}
\newfontfamily\hebrewfont{Arial}
\newfontfamily\hebrewfonttt{Arial}
\newfontfamily\hebrewfontsf{Arial}
\usepackage{showlabels}
\begin{document}
abc {\sffamily abc} {\ttfamily abc}
\end{document}

or

\documentclass{article}
\usepackage{polyglossia}
\setmainlanguage{hebrew}
\setmainfont{Arial}
\setsansfont{Arial}
\setmonofont{Arial}
\usepackage{showlabels}
\begin{document}
abc {\sffamily abc} {\ttfamily abc}
\end{document}

Or consider to use babel.

Answer 2

The other possibility is to adjust the font that showlabels is using, to one that evades the problem.

For example:

\usepackage{polyglossia}
\setmainlanguage{hebrew}
\setmainfont{Arial}
\usepackage{showlabels}
\renewcommand{\showlabelfont}{\textrm}

This successfully shows the label in the current text font. I expect more imaginative alternatives would work, too.