In your have a customization layer, add into one of your XSL files:
1) ...the internationalization namespace as an attribute
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:l="http://docbook.sourceforge.net/xmlns/l10n/1.0"
...
>
2) A parameter which points local.l10n.xml this very stylesheet
<xsl:param name="local.l10n.xml" select="document('')" />
3) A l:i18n element containing just the *changes* (or additions):
<l:i18n xmlns:l="http://docbook.sourceforge.net/xmlns/l10n/1.0">
<l:l10n language="de">
<l:gentext key="Index" text="Index"/>
<l:gentext key="index" text="Index"/>
<l:gentext key="setindex" text="Index"/>
<l:gentext key="SetIndex" text="Index"/>
</l:l10n>
</l:i18n>