The plan is to not have them included in the standard
distribution and not part of the release. Often a release
comes with changes to the English message bundles and we can't
afford to wait for all translations to be updated before a
release.
The repository would have a page listing the languages and
each language would have a maintainer listed (by Github
username). Ideally we'd also have the page automatically
updated with % keys translated and somehow figure out if it
needs to be updated (in the event the English translation has
changed).
Adding translations to the base theme should be made as
simple as possible. This will need some work to make as it's
not simple at the moment to add additional translations as it
requires a custom theme.