How does it work for existing translations for existing packages? Do they have to get manually translated again or... how will that work. I've got multiple Add-Ons on the marketplace which have translation files, but they don't get autoloaded it seems.

You can contact one of the translators of the language teams for which you have translations - seehttps://translate.concrete5.org/teams... - or I can upload them for you. In this latter case I won't be able to approve the translations (I don't know every language;).
BTW unapproved translations too will be included in the language files, they are simply in an "approval pending" state, and they can be modified by simple translators.
Approved strings can instead be modified only by the translation team coordinators.

This is fantastic, thank you mlocati.