One of our clients reported that the default output encoding for Docbook to HTML is ISO 8859-1 which is not suitable at all for other languages with extended char sets like Russian:
https://www.oxygenxml.com/forum/viewtopic.php?f=6&t=14812&p=43711#p43711
Maybe the default language for HTML (and also for HTML chunk) should be changed to be UTF-8 as UTF-8 is already used as the default language for XHTML.
Linking to the discussion Bob Stayton started on the Docbook Apps list:
https://lists.oasis-open.org/archives/docbook-apps/201708/msg00007.html
One more thing I discovered and added on the same forum thread:
https://www.oxygenxml.com/forum/viewtopic.php?f=6&t=14812&p=43711#p43722
Even if you set UTF-8 encoding by default Saxon 6 will still escape all special characters for "html" output. It has a special "saxon:" namespaced attribute which can be set on the xsl:output to avoid this but for some reason the attribute did not work when I tested it on the Docbook XSLTs.