diff --git a/cpdfua.ml b/cpdfua.ml
index bd1520a..e448770 100644
--- a/cpdfua.ml
+++ b/cpdfua.ml
@@ -348,6 +348,11 @@ let matterhorn_09_008 st st2 pdf =
 let matterhorn_10_001 _ _ pdf =
   unimpl ()
 
+(* Not clear what to do for 001...005 here - if the top-level /Lang is present,
+   that rules all and is sufficient. So how could these then fail? Perhaps they
+   mean if an intervening one has "" as the lang - i.e unknown. Or, if it is
+   not on the ISO 3066 registry list? *)
+
 (* Natural language for text in page content cannot be determined. *)
 let matterhorn_11_001 _ _ pdf =
   unimpl ()
@@ -367,7 +372,14 @@ let matterhorn_11_004 _ _ pdf = unimpl ()
 let matterhorn_11_005 _ _ pdf = unimpl ()
 
 (* Natural language for document metadata cannot be determined. *)
-let matterhorn_11_006 _ _ pdf = unimpl ()
+let matterhorn_11_006 _ _ pdf =
+  (* Per 2008 14.9.2.1-2, document metadata language is simply determined by
+     the top-level /Lang. In theory, one could omit the top-level /Lang and use
+     xml:lang everywhere in the metadata, but we haven't seen an example which
+     omits top-level /Lang so this will do for now. *)
+  match Pdf.lookup_chain pdf pdf.Pdf.trailerdict ["/Root"; "/Lang"] with
+  | Some (Pdf.String "") | None -> merror_str "No top-level /Lang"
+  | Some _ -> ()
 
 (* <Figure> tag alternative or replacement text missing. *)
 let matterhorn_13_004 _ st2 pdf =