langString datatype ignored by Virtuoso

[rogargon]

In SPARQL 1.0, the DATATYPE function was not defined for literals with a language tag. Therefore, an unextended implementation would raise an error when DATATYPE was called with a literal with a language tag. Operator extensibility allows implementations to return a result rather than raise an error. SPARQL 1.1 defines the result of DATATYPE applied to a literal with a language tag to be rdf:langString. However, Virtuoso is returning an empty string.