LaTeX template: set \babelfont...

when `mainlang` and a lang are specified and pdflatex is not being used.
This is needed for good results in Arabic and doesn't seem to hurt
for other languages we've tried.

Closes #8538.

data/templates/default.latex

@@ -343,6 +343,12 @@ $if(lang)$
 \fi
 $if(babel-lang)$
 \babelprovide[main,import]{$babel-lang$}
+$if(mainfont)$
+\ifPDFTeX
+\else
+\babelfont[$babel-lang$]{rm}{$mainfont$}
+\fi
+$endif$
 $endif$
 $for(babel-otherlangs)$
 \babelprovide[import]{$babel-otherlangs$}