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The backports for bug 1654901 include this patch:

| From: Igor Gnatenko <i.gnatenko.brain>
| Date: Wed, 21 Nov 2018 14:59:19 +0100
| Subject: [PATCH 1/3] build: make haveRichDep aware of other fields allowed to
|  have them
| 
| Recommends/Suggests/Supplements/Enhances can also have rich deps,
| so we need to extend the check. Or remove it.

This causes glibc-langpack-* packages to have rpmlib(RichDependencies) <= 4.12.0-1 dependencies, which in turn causes mock to to bail out during the bootstrap chroot setup:

Start(bootstrap): cleaning dnf metadata
Finish(bootstrap): cleaning dnf metadata
INFO: enabled HW Info plugin
Mock Version: 1.4.21
INFO: Mock Version: 1.4.21
Start(bootstrap): yum install
Failed to set locale, defaulting to C
[…]
ERROR You need to update rpm to handle:
rpmlib(RichDependencies) <= 4.12.0-1 is needed by glibc-langpack-zu-2.28-97.el8.x86_64

You should be able to use Mock using
  https://github.com/rpm-software-management/mock/wiki/Feature-container-for-bootstrap

However, IMO the use of RichDeps in glibc (or anything in @buildsys group) would be nice to postpone till death of RHEL7.

Folks, calm down. That patch was mistakenly included in the backport. I've reverted it and building now.

The problematic patch reverted, and thus issue fixed, in rpm-4.14.2-35.el8.

Thanks Florian for reporting!