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api.php?action=sitematrix&format=xml shows (excerpt)

<api>
  <sitematrix count="784">
    <specials>
...
      <special url="http://beta.wikiversity.org" code="betawikiversity" />
...
    </specials>
... 
    <language code="en" name="English">
      <site>
        <site url="http://en.wikipedia.org" code="wiki" />
        <site url="http://en.wiktionary.org" code="wiktionary" />
        <site url="http://en.wikibooks.org" code="wikibooks" />
        <site url="http://en.wikinews.org" code="wikinews" />
        <site url="http://en.wikiquote.org" code="wikiquote" />
        <site url="http://en.wikisource.org" code="wikisource" />
        <site url="http://en.wikiversity.org" code="wikiversity" />
      </site>
    </language>
...
  </sitematrix>
</api>

The tag within <language> should be <sites> instead of <site> like <specials> is.