luceos The primary reason is that extension developers aren't able to review all languages
This. Just no knowledge of foreign languages.
For example, in my extension flarum-profiles, someone did a PR for polish language and I don't know any polish (or french (ok a bit, but you get the point)) and it's not up-to-date anymore (it's missing a translation key because well, i don't know the translation), so I can't really flawlessy translate every translation key. The maintainers of language packs know better than me,
Qia Flarum language pack's files will be unnecessarily large for those who don't use any extensions which are not the default built-in extensions.
It's already large. You got all those core translation keys + all the official extensions. A few more keys won't hurt.
Qia The maintainer of a Flarum language pack can not guarantee that these translations are up-to-date (there are too many third-party extensions, so it's a large workload).
Neither the extension maintainer (even less than the language pack maintainer) - like i said above.
It just doesn't fit in an extension, while you have language packs. Language packs contain translations. Extensions shouldn't (only the english translation)
By saying it should be in the extension, then why didn't you do pull requests to the official extensions with those translation keys in stead of making the french translation pack?