Yeah, "make" itself fails, too, see bug #37491.

I'm not that knowledgeable with po strings but I think it added a
msgstr that's broken in that the number of format string parts is
different from msgid_plural.

I've edited po/guix/en@quot.po and po/guix/en@boldquot.po (which "git log"
doesn't work on) in order to work around the problem, something like

#: guix/scripts/pull.scm:307
#, scheme-format
msgid "  ~*One new channel:~%"
msgid_plural "  ~a new channels:~%"
msgstr[0] ""                          <---- edited
msgstr[1] ""                          <---- edited

but since I have no idea of what I'm doing there I've not committed it.

Hmm... https://www.gnu.org/software/gettext/manual/html_node/Translating-plural-forms.html