Hi! TL;DR the warning output is from libxml2 and the issue is caused by content of file /usr/share/docbook2X/xslt/common/text/l10n-set.xml that is indeed part of package docbook2x.
I have filed a report for this upstream with both a detailed analysis and a patch at https://sourceforge.net/p/docbook2x/bugs/25/ . I have pushed that very patch in Gentoo just now. Would be cool to stop seeing that warning again and again also in Debian and Ubuntu. Thank you! Best, Sebastian
