Set mainlang variable in context writer.

This parallels behavior of latex writer.  Mainlang is the last
of a comma-separated list of languages in lang.

src/Text/Pandoc/Writers/ConTeXt.hs

@@ -77,6 +77,8 @@ pandocToConTeXt options (Pandoc (Meta title authors date) blocks) = do
                  , ("title", titletext)
                  , ("date", datetext) ] ++
                  [ ("number-sections", "yes") | writerNumberSections options ] ++
+                 [ ("mainlang", maybe "" (reverse . takeWhile (/=',') . reverse)
+                                (lookup "lang" $ writerVariables options)) ] ++
                  [ ("author", a) | a <- authorstext ]
   return $ if writerStandalone options
               then renderTemplate context $ writerTemplate options

templates

@@ -1 +1 @@
-Subproject commit d709b6709b7ca2eef8100f5ed4779a5028f3f14f
+Subproject commit 7c4816983e652dddbd15adcfffa13823adec944e