OK. Until libzypp is fixed, I think I can work around the problem:
In libyui-{qt,ncurses}-pkg I can temporarily set ignoreAlreadyRecommended() in
the handler of the Languages view, around the calls of
addRequestedLocale/removeRequestedLocale:
NCPkgLocaleTable::toggleStatus
https://github.com/libyui/libyui-ncurses-pkg/blob/6ee2566a0590e119e9a012bd593d96e1c5cadc9f/src/NCPkgFilterLocale.cc#L186
YQPkgLangListItem::setStatus
https://github.com/libyui/libyui-qt-pkg/blob/9363013550cbb6b410211498428e8b8d779c1de3/src/YQPkgLangList.cc#L255
I'll try how it works out.