The idea behind "langpacks" is to separate translations or language specific content into subpackages in the case that the size of the files is huge or the package is part of a core image that should be minimal.
Subpackages that exist solely to contain additional language translations or content must be named in the form -langpack-, where is the name of the package that the langpacks belong to and is a valid language code from /usr/share/xml/iso-codes/iso_639_3.xml or from /usr/share/i18n/locales/. Specifically, the langcode value used in the package name must agree with the langcode identifier used in the directory path by upstream for the translation or language files.
The langpack ecosystem does not need any procedural logic in the form of plugins. Instead it takes advantage of the weak and rich dependency features provided by RPM. The necessary dependencies are computed by the package manager (DNF or PackageKit) so it is essential to include the following Supplements: tag relation in the langpack package definition in the spec file:
Supplements: (%{name} = %{version}-%{release} and langpacks-`)`
Example
Suppose you have a package with following spec file:
Name: php-horde-Horde-Perms
...
%files -f Horde_Perms.lang
/usr/share/pear-data/Horde_Perms/locale/
...
In order to create langpacks for each language (/usr/share/pear-data/Horde_Perms/locale//LC_MESSAGES/Horde_Perms.mo translations), you would need to define a new subpackage for each package. The snippet below shows a macro for automating the definition of langpacks, along with the definition of langpacks for bs and cs languages. This would replace the lines shown above. Note that this macro definition is specific to the example package; you will need to modify it as appropriate for your package.
Name: php-horde-Horde-Perms
...
%define lang_subpkg() \
%package langpack-%{1}\
Summary: %{2} language data for %{name}\
BuildArch: noarch\
Requires: %{name} = %{version}-%{release}\
Supplements: (%{name} = %{version}-%{release} and langpacks-%{1})\
\
%description langpack-%{1}\
%{2} language data for %{name}.\
\
%files langpack-%{1}\
%{_datadir}/pear-data/Horde_Perms/locale/%{1}/
+ %lang_subpkg bs Bosnian+
%lang_subpkg cs Czech
...
%files
...