Set the following options to values appropriate for DocBook:
[HTMLOptions]
; UseDOCTYPE = Yes (default) or No (when writing DocBook entity files)
UseDOCTYPE = No
; UseXMLRoot = Yes (default) or No (when writing DocBook entity files)
UseXMLRoot = No
; XHLangAttr = xml:lang (default, set as needed)
XHLangAttr = xml:lang
When [DocBookOptions]WriteBookFile=Yes, DITA2Go sets UseDOCTYPE and UseXMLRoot to No for chapter entity files; see §26.2.2 Specifying DocBook output options.