The dns/libidn2 fails to build (in the doc/ subdirectory), when p5-Locale-gettext-1.07 is not installed. I don't know, why poudriere is not catching this -- perhaps, LANG needs to be set during build to trigger the error... (I have it set to uk_UA.KOI8-U.)
If you build misc/help2man with NLS enabled, p5-Locale-gettext should be installed already. If you build misc/help2man without NLS enabled, /usr/local/bin/help2man should not have "use Locale::gettext qw(gettext);" line. Therefore, I cannot reproduce this problem. "cd /usr/ports/dns/libidn2 && make LANG=uk_UA.KOI8-U" succeed with either configuration.