Org reader: parse #+LANGUAGE into lang metadata field

Fixes: #6845

doc/org.md

@@ -22,7 +22,8 @@ The following export keywords are supported:
 - EMAIL: author email address; passed as plain-text metadata
   field `email`, but not used by any default templates.
 
-- LANGUAGE: currently unsupported; use `#+LANG:` instead.
+- LANGUAGE: document language; included as plain-text metadata
+  field `lang`. The value should be a [BCP47 language tag].
 
 - SELECT_TAGS: tags which select a tree for export. Currently
   *unsupported*.
@@ -36,6 +37,8 @@ The following export keywords are supported:
   defaults to stdout unless a target has to be given as a command
   line option.
 
+[BCP47 language tag]: https://tools.ietf.org/html/bcp47
+
 Format-specific options
 -----------------------
 

src/Text/Pandoc/Readers/Org/Meta.hs

@@ -93,6 +93,8 @@ keywordHandlers = Map.fromList
   , "institute" ~~> lineOfInlines `parseThen` collectLines "institute"
   -- topic keywords
   , "keywords" ~~> lineOfInlines `parseThen` collectLines "keywords"
+  -- document language
+  , "language" ~~> fmap pure anyLine `parseThen` B.setMeta "lang"
   -- LaTeX-specific export settings
   , "latex_class" ~~> fmap pure anyLine `parseThen` B.setMeta "documentclass"
   , "latex_class_options" ~~>

test/Tests/Readers/Org/Meta.hs

@@ -112,6 +112,10 @@ tests =
       "#+institute: ACME Inc." =?>
       Pandoc (setMeta "institute" ("ACME Inc." :: Inlines) nullMeta) mempty
 
+    , "Document language" =:
+      "#+LANGUAGE: de-DE" =?>
+      Pandoc (setMeta "lang" (MetaString "de-DE") nullMeta) mempty
+
     , testGroup "LaTeX"
       [ "LATEX_HEADER" =:
         "#+latex_header: \\usepackage{tikz}" =?>