Adding the descriptions in two languages :

<UPDATE>
  <MEDIA ID="12">
    <DESCRIPTIONS>
      <DESCRIPTION>
        <LANGUAGEID>eng</LANGUAGEID>
        <TITLE>Taking the descendants</TITLE>
        <CUSTOM><Icon>/media/icons/icon1.gif</Icon></CUSTOM>
      </DESCRIPTION>
      <DESCRIPTION>
        <LANGUAGEID>fre</LANGUAGEID>
        <TITLE>Prendre les descendants</TITLE>
        <CUSTOM><Icon>/media/icons/icon1.gif</Icon></CUSTOM>
      </DESCRIPTION>
    </DESCRIPTIONS>
  </MEDIA>
</UPDATE>

The only mandatory field is the LANGUAGEID. If you don't put a LANGUAGEID, the description will not be added.