ox-latex: Allow AUTO argument to org-latex-guess-babel-language

* ox-latex.el (org-latex-guess-babel-language): replace AUTO with
  language if AUTO is the option of the LaTeX package Babel.
(org-latex-classes): updated documentation with respect
to `org-latex-guess-babel-language'.

TINYCHANGE

lisp/ox-latex.el

@@ -255,12 +255,17 @@ to \\providecommand, and then place \\usepackage commands based
 on the content of `org-latex-packages-alist'.
 
 If your header, `org-latex-default-packages-alist' or
-`org-latex-packages-alist' inserts
-\"\\usepackage[AUTO]{inputenc}\", AUTO will automatically be
-replaced with a coding system derived from
-`buffer-file-coding-system'.  See also the variable
+`org-latex-packages-alist' inserts \"\\usepackage[AUTO]{inputenc}\",
+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.
+
 The sectioning structure
 ------------------------
 
@@ -910,6 +915,10 @@ Insertion of guessed language only happens when Babel package has
 explicitly been loaded.  Then it is added to the rest of
 package's options.
 
+The argument to Babel may be \"AUTO\" which is then replaced with
+the language of the document or `org-export-default-language'
+unless language in question is already loaded.
+
 Return the new header."
   (let ((language-code (plist-get info :language)))
     ;; If no language is set or Babel package is not loaded, return
@@ -918,16 +927,19 @@ Return the new header."
 	    (not (string-match "\\\\usepackage\\[\\(.*\\)\\]{babel}" header)))
 	header
       (let ((options (save-match-data
-		       (org-split-string (match-string 1 header) ",")))
+		       (org-split-string (match-string 1 header) ",[ \t]*")))
 	    (language (cdr (assoc language-code
 				  org-latex-babel-language-alist))))
-	;; If LANGUAGE is already loaded, return header.  Otherwise,
-	;; append LANGUAGE to other options.
-	(if (member language options) header
-	  (replace-match (mapconcat 'identity
-				    (append options (list language))
-				    ",")
-			 nil nil header 1))))))
+	;; If LANGUAGE is already loaded, return header without AUTO.
+	;; Otherwise, replace AUTO with language or append language if
+	;; AUTO is not present.
+	(replace-match
+	 (mapconcat (lambda (option) (if (equal "AUTO" option) language option))
+		    (cond ((member language options) (delete "AUTO" options))
+			  ((member "AUTO" options) options)
+			  (t (append options (list language))))
+		    ", ")
+	 t nil header 1)))))
 
 (defun org-latex--find-verb-separator (s)
   "Return a character not used in string S.