/xsl/common/fr.xsl has size 0 so we're unable to build french doc without finding another fr.xml from another version. This version should be removed from the portage tree I think.
the exact filename in the tarball is "docbook-xsl-1.62.4/common/fr.xml" sorry for typo
You are correct, it has size 0. This is fixed in 1.64.1, so I'm turning this bug into a request for that version.
Fixed in the newest version 1.65.1, thanks for your report.