Adding 1.38 release per Timo's request, but I do not believe this issue is significant enough to be a blocker given it impacts one message on the main page which can easily be overriden.

I do agree the current state is not perfect but that is mostly due to a lack of information and all alternative solutions also have their imperfections.

I repeat as I said before:

In T301203#7800970, @Krinkle wrote:

Many of these are now using a different label than the 'mainpage' and other 'mainpage-' messages. These should very rarely differ, if ever.

za.json

 mainpage: "Yiebdaeuz",
 mainpage-description: "Yiebdeauz",
 mainpage-nstab: "Yiebdaeuz",
 tooltip-n-mainpage: "Raen Yiebdaeuz",
- "nstab-mainpage": "Yiebdaeuz"
+ "nstab-mainpage": "Feanzcieng

nb.json

 mainpage: "Forside",
 mainpage-description: "Forside",
 mainpage-nstab: "Forside",
 tooltip-n-mainpage: "Forside",
+ "nstab-mainpage": "Hovedside"

Both these and various other language files are now out of sync with the rest of the translation. This improves nothing and was completely unnecessary. This change took a single message and blindly copied WMF-local translations that were different from their default into MediaWiki core. The result is that language editions that choose to customise their main page name, now use that custom name for 1 of the hundreds of messages in core for that language. These are longstanding overrides that translators are generally well-aware of and have decided years ago not to bring into the software. In the case of no.json specifically, I suspect that it is simply the case of 1 message having been forgotten to be updated.

For example, no.wikipedia.org renamed their main page from Hovedside to the MW standard of Forside, and they simply forgot to change/remove this one interface message.

I don't think you made these changes intentionally. It was clearly automated without reviewing the target file content. That makes sense, it's a pretty large file at several thousand lines of foreign language content. It's not meant to be edited by hand in this way. Such changes, as you point out, are meant to go through translatewiki.net.

With the above diff context provided on-screen, I think the mistake is obvious, wouldn't you agree?

Also, I hadn't yet mentioned, the change caused the nstab-mainpage message key to be added to many files that already set the mainpage-nstab key. This doesn't make sense since one is deprecated. They must not both be in the localisation system. There is a process for renaming messages, documented on mediawiki.org, which you also didn't follow.

I believed that Translatewiki would be the best platform to have these discussions with the experts in those languages, but Timo disagreed.

I've made no such disagreement. I in fact stated the opposite, both above and on the change: This change should not have been made in Gerrit. Your change to individual localisation files was inappropiate and should have instead been a suggestion to translatewiki.net, where such suggestion would (or will still be) most likely rejected, or if contributed directly, reverted during translate review. (Making the change through the software, however, bypassed this review phase, I think?)

I'm not sure how this should be resolved, but I don't think it should block the 1.38 release.