Why does \setdefaultlanguage modify my fontfamily?

Question

Please see this example code

\documentclass{article}
\usepackage{polyglossia}
\setdefaultlanguage[spelling=new]{german}

\renewcommand*\familydefault{\sfdefault}
\begin{document}
test
\end{document}

test is written in \rmfamily not \sffamily. But when you kill the third line with \setdefaultlanguage the correct font is used.

Why? What is behind that?

Answer 1

This is arguably a bug in polyglossia. Examining the polyglossia.sty one sees it has a concept of \familytype. When the user has selected a language, the font selection is influenced by it this tests the value of \familytype.

There is some code to set during package loading \familytype:

\expandafter\ifx\familydefault\sfdefault
  \def\familytype{sf}
\else\expandafter\ifx\familydefault\ttdefault
  \def\familytype{tt}
\else
  \def\familytype{rm}
\fi\fi

This has two issues:

1. arguably it should be done at begin document, not during package loading, to allow user's redefinitions of \familydefault anywhere in the preamble (and personally I would wish to also be able to define it in the document body, it is not immediately clear to me why polyglossia needs its own \familytype rather than \familydefault, restricting the efficiency of the latter).

2. the \expandafter\ifx\familydefault constrains user's definition of \familydefault to be done in a certain way, actually it constrains it to be in the style of the OP: \renewcommand*\familydefault{\sfdefault}. In particular \let\familydefault\sfdefault would lead to unexpected outcome. But ideally the full expansion should be used.

Thus there are two work-arounds: first one is to do \renewcommand*\familydefault{\sfdefault} before loading polyglossia, alternative is to do \def\familytype{sf} at the end of the preamble.

First method:

\documentclass{article}

\renewcommand*\familydefault{\sfdefault}
\usepackage{polyglossia}
\setdefaultlanguage[spelling=new]{german}
\def\test{test}
%\def\familytype{sf}
% \tracingmacros1
\begin{document}
\test
\end{document}
% Local Variables:
% TeX-engine: xetex
% End:

Second method:

\documentclass{article}

%\renewcommand*\familydefault{\sfdefault}
\usepackage{polyglossia}
\setdefaultlanguage[spelling=new]{german}
\def\test{test}
\def\familytype{sf}
% \tracingmacros1
\begin{document}
\test
\end{document}
% Local Variables:
% TeX-engine: xetex
% End:

Answer 2

Adding \AtBeginDocument{\normalfont} in the preamble anywhere after \setdefaultlanguage will also suffice as a workaround

\documentclass{article}
\usepackage{polyglossia}
\setdefaultlanguage[spelling=new]{german}
\AtBeginDocument{\normalfont}

\renewcommand*\familydefault{\sfdefault}
\begin{document}
test
\end{document}