slides XSL: localizing the localization

Excuse about my ignorance, but how do I extend a localization locally?
For slides to work in French, I added 

  <xsl:param name="local.l10n.xml" select="document('')"/> 
  <i18n xmlns="http://docbook.sourceforge.net/xmlns/l10n/1.0">
    <l:l10n xmlns:l="http://docbook.sourceforge.net/xmlns/l10n/1.0" language="fr">
      <l:gentext key="Continued" text="(Continué)"/>
  ...

to my XSL customization layer. It works, but it replaces - rather than
adds to - the existing localization in the slides style sheet, meaning
that the English option is lost. There surely is a way to do _add_
languages...?

Thanks,
Justus

-- 
Justus H. Piater, Ph.D.         http://www.montefiore.ulg.ac.be/~piater
Institut Montefiore, B28        Phone: +32-4-366-2279
Université de Liège, Belgium    Fax:   +32-4-366-2620