Problem/Motivation
When you run an installation creating a site in (default) English and add a language afterwards, it is more work than installing in another language and adding English later.
Proposed resolution
Whenever you activate a language, look up the core .po files and add them to the site or (second best) point to the location where to find these files.
Remaining tasks
TBD
User interface changes
TBD
API changes
Not sure
Description
Reproduce
- Install D8 in English and add Dutch afterwards
- Install D8 in Dutch (Nederlands) and add English afterwards
In the first case you need to add the Dutch language files. In the second case Drupal finds the Dutch language files and adds them for you. But not for the third language....