Adding lang to External

Author: glenrobson

schema/v4/External.json

@@ -14,6 +14,12 @@
             "format": {
                 "$ref": "properties.json#/$defs/format"
             },
+            "language": {
+                "type": "array",
+                "items": {
+                    "$ref": "properties.json#/$defs/BCP47"
+                }
+            },
             "profile": {
                 "type": "string"
             }