It is documented as 'needed for the libvirt-guests initscript'. However, that initscript doesn't exist anymore, and the systemd service file shows no signs of needing gettext.
The systemd service file calls into the /usr/libexec/libvirt-guests.sh shell script, and THAT file DOES deppend on gettext. Basically, when we moved from initscripts to systemd, we migrated what used to be the init script into a standalone shell script that both initscripts and systemd could reuse. As such, I'm closing this as not a bug; the dependency on gettext is intentional.
That said, I did make this upstream tweak to fix the comment. commit 9615ca59c6ec41fc2f41b2b89087ff1cacfd1dec Author: Eric Blake <eblake> Date: Tue May 14 20:14:34 2013 -0600 spec: fix outdated comment https://bugzilla.redhat.com/show_bug.cgi?id=963016 points out that we don't use initscripts by default on Fedora any more. * libvirt.spec.in (Requires): Better explanation of gettext. Signed-off-by: Eric Blake <eblake>