The multi-lingual multi-domain problem

Background

In a multi-lingual install, the schema is changed to add columns for all translatable fields.
One column is added per language.
The list of languages is stored in civicrm_domain.locales
For unknown historical reasons, locales was only populated for domain 1. It was thus treated as a "master domain" although there doesn't appear to be any documentation of this.
So in a multi-domain setup, domain 2 would inherit its language settings from domain 1.

Having civicrm_domain.locales be populated for domain 1 but null for domain 2+ is confusing to devs and leads to crashes when relying on that value from the current domain.
Bug reported in #1800 (closed)

Ensure all domains have locales populated when enabling multilingual: https://github.com/civicrm/civicrm-core/pull/17733
An upgrade script to ensure locales is the same for all domains: https://github.com/civicrm/civicrm-core/pull/17738

Once the relatively easy stuff is solved by merging the fixes above, we might consider:

What happens if the admin desires one domain to be single-language and another to be multi?
When reverting from multi-lingual back to single, are all domains affected?
What if different multi-lingual domains want to support different languages?
Is all of the above completely unreasonable and multilingual settings should just always be the same across domains?

Edited Jul 21, 2020 by colemanw

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information