With the release of RDF 1.1 spec and Jena 3 software, the relationship between xsd:datatype and lang has changed.
Previously a literal could have one or the other, but not both. Presence of a lang tag presumed a datatype of string.
In 1.1, langString can explicitly indicate that a literal value contains text that is to be rendered using a language. A language tag can be added to a langString declaration to indicate the language of the langString.
string can be used as before, but could be reserved for string that are not text – such as identifiers.
VIVO should develop documentation regarding best practices for the use of xsd:string, xsd:langString and lang tags.