Usable and used languages should be (better) synchronized

[rk-for-zulip]

At present, we have mostly-complete translations for Finnish, Korean, and a few other languages that aren't present in the language selection screen. We also have some currently-unusable translations being loaded in messages.js, along with their locales in locale.js.

The sets of usable, selectable, and imported languages should be synchronized; and, more generally, it should be easy to keep them synchronized in the future.

[gnprice]

Indeed!

#3896 is merged; and see the WIP PR #3897, which will do a long-overdue sync. Even after that, it will be useful to have more automation so we more easily do better at keeping these synced in the future. (It won't be all automation, though; there are a few human choices to make, mainly in the best names to use for some languages.)

See also the bit of docs added in 6775b4f.

[gnprice]

#3897 has been merged, and #3959 which took another step to get these better in sync.

#3915 is an open PR to automate keeping them in sync for the future.