Again, I think that include translations of extensions into the core translation is a bad idea. It will unnecessary increase the file weight, and I think the default package of Flarum should be as light as possible, without useless/unused lines of code.
Also, I think extension developers should manage translations on their own, not Flarum developers. See an extension as a third-party product, Flarum should not manage them (unless the extension do not meets the coding guidelines or have security issue of course). Don't worry, if an extension is popular, translators will be there for it.
To illustrate my words above:
Example of a core translation architecture:
system/languages/(iso)/file1.yml
system/languages/(iso)/file2.yml
system/languages/(iso)/file3.yml
...
Example of the content of the "Hello World" extension package:
system/languages/(iso)/extensions/helloworld.yml
system/extensions/helloworld/file1.php
system/extensions/helloworld/file2.php
system/extensions/helloworld/file3.js
...
(iso) as ISO code, like fr, en-gb, en-us, de, pt, pt-br, etc.
Also, amdad got a point. Translations should be independant from the default package. I know it's more user-friendly to have them in core, but it will be a pain in the ass for developers to handle them (like abandoned translations, outdated translations, or translators asking some more time which delay the software release, etc.). It's sad to say, but you can't just trust your translators to update their translations with the latest version. I'm a former Translations Manager on the phpBB software, and everybody are not as serious as you want to. Do not forget it's a free and volunteer work, so people are doing that on their free time. I always kept my translations up-to-date, released them at the second a new software version is available, but that's because I'm passionate and involved. But trust me, it's not the case of everyone, some of them just want a software in their language in a specific priod of time, and do not reappear again, or very rarely.
Is it possible to just improve the admin panel to install and delete translations with a one-click feature? I'm not a developer, so I don't know how this will be hard (or heavy) to implement. I know WordPress handle one-click installations of extensions, styles and translations very nicely. This will solve the "user-friendly" thing.