This will break on the update to RDF-1.1, where the datatype is always populated, but not in the way the OWL WG suggested. This will occur when using Sesame-2.8 which contains this update. Instead of the OWL WG path of using RDF_PLAIN_LITERAL for both plain literals and language-tagged literals, the RDF-1.1 group went for two different datatypes, xsd:string and rdf:LangString, for the two subsets of the rdf:PlainLiteral space, respectively.

If, in OWLAPI, both of these datatypes are to be aggregated under rdf:PlainLiteral, then there needs to be a check here to convert both xsd:string and rdf:LangString to rdf:PlainLiteral. Alternatively, xsd:string could be left as is, and rdf:PlainLiteral would only practically be used for language-tagged literals.