Thanks for the quick reply.
If this is intended, then how you do you do minor grammar or typo corrections for English, without affecting all other languages and having to redo all the other translations?
Apartment and Flat was just an arbitrary example of changing the english (native) loca, unrelated to en-GB.
The edit from Apartment->Apartment to Apartment->Flat was done in the Localization Dashboard. Will the Localization Dashboard remain an intended place to edit localization, besides importing and exporting .po files?
So to summarize:
We have the 4 levels: key → source_text → native_translation → other_translation.
In the English (native) .archive, key plays no role, since it only contains source_text → native_translation.
In the Spanish .archive, key and source_text play no role, since it only contains native_translation → other_translation.
source_text is actually not used as a common source for all languages, just once during the creation of the English Loca.