If you want to use xml:lang, xml:space and xml:base in your own schema
you need to import following namespace.
<import namespace="http://www.w3.org/XML/1998/namespace" schemaLocation="xml_tva.xsd"/>
However stylus studio requires that the prefix xml is bound to this namespace by adding xmlns:xml="http://www.w3.org/XML/1998/namespace"
which is incorrect behaviour
See
[0] http://www.w3.org/TR/xmlschema-1/#normative-schemaSchema
[1] http://www.w3.org/TR/1999/REC-xml-names-19990114/#nsc-NSDeclared
Or the discussion on xml-dev with subject
"which xml schema tools do it right concerning including attributes xml:lang and xml:space"