See example: https://meta.wikimedia.org/w/index.php?title=Special:Translate&group=page-Wikipedia+15&action=page&filter=&language=zh-hans >> "Translate in zh please."
But https://meta.wikimedia.org/wiki/Wikipedia_15/zh-hans is still exists.
How?
That's how - https://meta.wikimedia.org/w/index.php?title=Translations:Wikipedia_15/Page_display_title/zh-hans&action=edit
Etc., item by item.
There are related bugs in the interface that causes the error message not to show when switching languages.
Is this a feature request to hard block creation of such pages similar to what happens to unknown messages in Translations namespace?
Yes. I believe that creation and editing messages with the langcode, which is disabled in a related setting ($wgTranslateBlacklist ?), should be blocked.
CentralNotice currently depends on the user being able to save to the CNBanner namespace's /en pages so doing this would partially break it at Meta because 'en' is blacklisted for all groups on Wikimedia. That blacklist entry shouldn't actually be needed and manual source page edits should actually be directly prevented on Translate but even then we would need to add exceptions for CNBanner.
Change 306015 had a related patch set uploaded (by Glaisher):
Remove English for all groups from $wgTranslateBlacklist
That blacklist entry shouldn't actually be needed and manual source page edits should actually be directly prevented on Translate
AFAIK we fixed that in T89637: Automatically blacklist the page's source language without manual $wgTranslateBlacklist.
Change 306015 merged by jenkins-bot:
Remove English for all groups from $wgTranslateBlacklist
Mentioned in SAL [2016-08-23T13:21:01Z] <hashar@tin> Synchronized wmf-config/CommonSettings.php: Remove English for all groups from $wgTranslateBlacklist T124013 (duration: 00m 54s)
Change 306024 had a related patch set uploaded (by Glaisher):
Use getUserPermissionsErrorExpensive hook for preventing translations to non-translatable languages
Change 307054 had a related patch set uploaded (by Glaisher):
Prevent saving translations when if it matches $wgTranslateBlacklist
Change 307054 merged by jenkins-bot:
Prevent saving translations if it matches $wgTranslateBlacklist