LaTeX template: Add pdflang to hypersetup if lang is set.

Closes #5443.

data/templates/default.latex

@@ -205,6 +205,9 @@ $endif$
 $if(author-meta)$
   pdfauthor={$author-meta$},
 $endif$
+$if(lang)$
+  pdflang={$lang$},
+$endif$
 $if(subject)$
   pdfsubject={$subject$},
 $endif$

test/writers-lang-and-dir.latex

@@ -37,6 +37,7 @@
 \IfFileExists{xurl.sty}{\usepackage{xurl}}{} % add URL line breaks if available
 \IfFileExists{bookmark.sty}{\usepackage{bookmark}}{\usepackage{hyperref}}
 \hypersetup{
+  pdflang={en},
   hidelinks,
   pdfcreator={LaTeX via pandoc}}
 \urlstyle{same} % disable monospaced font for URLs