SES-1803 : Avoid returning true from isDatatypeLiteral for
rdf:langString
SES-1803 : Simplify QueryEvaluationUtil.compatibleArguments now that all
plain and simple literals have xsd:string as their datatype
SES-1803 : Experimentally remove isLanguageLiteral from isStringLiteral
to verify its behaviour and fix the locations where it broke