type for srcLang and trgLang not declared in the core xsd

When editing an XLIFF file by hand I realized that file

<xliff version="2.1" xmlns="urn:oasis:names:tc:xliff:document:2.0"
srcLang="" trgLang="">
<file id="f1">
<unit id="u1">
<segment id="s1">
<source>test</source>
<target>test</target>
</segment>
</unit>
</file>
</xliff>

does not report any error when validated against schemas. Note the
srcLang="" trgLang="".

The root cause seems to be in xliff_core_2.0.xsd:
<xs:attribute name="srcLang" use="required"/>
<xs:attribute name="trgLang" use="optional"/>
where type is not defined.