Add notes on Polyglossia support

* ox-latex.el (org-latex-classes): Update docstring.
* ORG-NEWS: Update.

etc/ORG-NEWS

@@ -401,6 +401,9 @@ subtree and creates one shifted, repeating clone.
 *** None-floating tables, graphics and blocks can have captions
 *** New time block for clock tables : ~untilnow~
 It encompasses all past closed clocks.
+*** Support for the ~polyglossia~ LaTeX package
+See the docstring of ~org-latex-classes~ and
+~org-latex-guess-polyglossia-language~ for details.
 ** Miscellaneous
 *** Strip all meta data from ITEM special property
 ITEM special property does not contain TODO, priority or tags anymore.

lisp/ox-latex.el

@@ -478,11 +478,15 @@ AUTO will automatically be replaced with a coding system derived
 from `buffer-file-coding-system'.  See also the variable
 `org-latex-inputenc-alist' for a way to influence this mechanism.
 
-Likewise, if your header contains \"\\usepackage[AUTO]{babel}\",
-AUTO will be replaced with the language related to the language
-code specified by `org-export-default-language', which see.  Note
-that constructions such as \"\\usepackage[french,AUTO,english]{babel}\"
-are permitted.
+Likewise, if your header contains \"\\usepackage[AUTO]{babel}\"
+or \"\\usepackage[AUTO]{polyglossia}\", AUTO will be replaced
+with the language related to the language code specified by
+`org-export-default-language'.  Note that constructions such as
+\"\\usepackage[french,AUTO,english]{babel}\" are permitted.  For
+Polyglossia the language will be set via the macros
+\"\\setmainlanguage\" and \"\\setotherlanguage\".  See also
+`org-latex-guess-babel-language' and
+`org-latex-guess-polyglossia-language'.
 
 The sectioning structure
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