I'd say it's a dblatex problem. DocBook 4.2 spec says
ID is an identifying string for the element. It must be unique at least within the document and must begin with a letter.
Nothing said here about a limitation to ASCII. And here the XML file clearly indicates that the encoding is UTF-8. In addition, `xmlstarlet val` says that the XML file is valid. Pandoc has an `--ascii` option to force ASCII output, but currently it only works for HTML output, not DocBook. This could be changed, but please submit an enhancement request to pandoc's own upstream bug tracker, https://github.com/jgm/pandoc/issues, rather than here.
