Looking at the packages, many of them now use:

%if !0%{?is_opensuse}
translation-update-upstream
%endif

This is an ugly divergence between SLE15 and Leap, as it forces splitting of
translation-update and translation-update-upstream packages for SLE15 and
Leap15, and doubles amount of work on translation updates. Disabling
translation-update-upstream makes impossible to do a build-time translation
update and forces post-installation updates). It makes translation-update for
SLE unusable for Leap, and it also increases size of installation image with
translations.

I am aware of a small increase of build time, but it makes possible to easily
update translations directly in the lang package without touching the sources.

Technical details:

translation-update-upstream: Package that contains new upstream translations
that can be used during the build time. It contains supplementary scripts to
get the update.

translation-update: Package that contains post-installation updates. The
tarball in this package is generated from translation-update-upstream using the
%prep log (if translation-update-upsteam succeeds, build-time translation
update is used, if it fails, post-installation translation update is used).
translation-update depends on a dedicated glibc patch.