For app-text/iso-codes-3.54, I had to implement a private analogue of l10n_find_plocales_changes because I needed to check the union of lists of .po files present in multiple directories. I would be nice if l10n.eclass supported this use case directly.
https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=6b7e7b1e66a024801b25dd2402d019630076d15b "l10n.eclass: Remove eclass"