UndeadLime welcome to Flarum!
1) The English language pack can definitely be used as an example and is the authoritative list of translation keys. Make sure you check the GitHub tags as the
master branch will contain translations for future features and might be missing translations for features that will be removed in the next Flarum version.
Copying another language pack is also a good starting point, just be respectful with the licensing/attribution if you decide to keep some already-translated strings or additional code/images that might be bundled in it.
2) Extiverse is a good place to get a rough popularity sort based on downloads https://extiverse.com/?sort=-downloads Just keep in mind those are lifetime statistics, so there are some obsolete/abandoned extensions in there that might not see much use anymore. Some have also been forked into new packages.
You could also take a look at which extensions do the other language packs translate. You can also look up on Extiverse which are the most downloaded language packs to get a starting point on that.
3) While we did some experiments regarding managed translations in the past, I believe that's currently on hold. I'll let someone else from the team comment on that though.