DocBook 1.78.1, file: common/titles.xsl, line 485ff:
[...]
<xsl:template match="answer" mode="title.markup">
<!-- answers don't have titles -->
<xsl:text>Answer</xsl:text>
</xsl:template>
<xsl:template match="qandaentry" mode="title.markup">
<!-- qandaentrys are represented by the first question in them -->
<xsl:text>Question</xsl:text>
</xsl:template>
[...]
These seem to be missing gentext calls to make them localizable. On the ML, Bob Stayton agreed that this was a bug, although it would rarely crop up, since both elements usually only use labels, but not titles.
Mailing list thread: http://markmail.org/thread/bcsyc5ztg56fkbz4 .
I checked in a fix for this. It should appear in the next snapshot build.