describe: add --lang back into external_lib_deps for backcompat

Signed-off-by: Ali Caglayan <alizter@gmail.com>

bin/describe/describe_external_lib_deps.ml

@@ -160,6 +160,14 @@ let to_dyn context_name external_resolved_libs =
 let term =
   let+ common = Common.term
   and+ context_name = Common.context_arg ~doc:"Build context to use."
+  and+ _ =
+    Arg.(
+      value
+      & opt (some string) None
+      & info [ "lang" ] ~docv:"VERSION"
+          ~doc:
+            "This argument has no effect and is deprecated. It exists solely \
+             for backwards compatibility.")
   and+ format = Describe_format.arg in
   let config = Common.init common in
   Scheduler.go ~common ~config @@ fun () ->