<Questions xmlns="http://asdf.asdf">
<Languages defaultLanguage="en">
<Language name="de"/>
<Language name="en"/>
</Languages>
<Question id="1">
<Text lang="en">Are you human?</Text>
</Question>
<Question id="2">
<Text lang="en">Do you have five fingers?</Text>
<Text lang="de">Hast du fünf Finger?</Text>
</Question>
</Questions>
It should be validated by XML-Schema that each "Question" has a "Text" entry for the languages defined in "Languages". With keyref the Text.lang attribute can be validated to be of the defined language name.
In the above example the validation should fail because question 1 lacks the "Text" element with the attribute lang="de".