datatype properties' range

Hi all,

in the RDF ontology, all datatype properties with textual content have 
range xsd:string.

I think using rdfs:Literal instead would be beneficial, as we could 
(optionally) add a language tag to those textual values.

I mention it because the API document allows to attach a language 
information to virtually any property value. This would be a nice way to 
encode this in RDF as well.

   pa