Moving discussion from https://discuss.flarum.org/d/22802-italian-language-pack-extended/10
luceos this has become the de facto standard recently. Splitting translations for core and extensions is easier to manage overall. In addition there's a bug that loads all translations into flarum even for extensions that aren't enabled. Splitting translations up helps a little.
It does not help at all: core + official extensions lacks a lot of features, you need to install at least several 3rd party extensions to provide some essential features. Core-only installations ale probably super rare.
I'm also not sure how maintaining two extensions may be simpler than maintaining only one. There is no case when it will save you some work - you need to do release anyway, and with two extensions you may need to do 2 releases instead of one. I've even seen bugs because of this separation - "core" and "extensions" language packs have translations for the same extension (one of them was outdated) and outdated one overwritten current one.
From user perspective this is just ridiculous that you need to install 2 language packs for the same language to translate your forum. I really hope that we get rid of this convention after moving forward with flarum/core2020 and moving English translations from English language pack to respective extensions.