Hello,
sorry for sending a comment after the review closed.
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.