Hi @Openchronicle, thanks for taking the time to report this!
Is this a public wiki that you could link to? Is it only that tab, or also other places?
Has this been brought up already on https://www.mediawiki.org/wiki/Project:Support_desk ?

Unfortunately, this concerns a private wiki. That problem isn't brought up on the support desk. Beside the ParserFunction extension there are no other extensions in use. Namespaces (or its handling) are not modified. Upgrade was done just by copying folder images and LocalSettings.php to the new MediaWiki 1.26.0 folder. And executing update.php of course.

Made a fresh installation of MediaWiki 1.26.0 some minutes ago. Same here: the main page tab is not translated.

I can't reproduce this. The tab is rendered by interface message mainpage-nstab.

Viewing my localhost/wiki/Main_Page?uselang=de shows "Hauptseite" instead of "Main Page", as expected.

The page title itself is still Main Page, but that's expected.

Tried another fresh installation (Language settings were set to "german" during installation wizard). Main page is never translated also with use of uselang param.
But MediaWiki:Mainpage-nstab is set to Main page...

Looks like that translation is not exported for 1.26. Maybe it was translated after the branch cut?

https://github.com/wikimedia/mediawiki/blob/REL1_26/languages/i18n/en.json#L304

	"nstab-category": "Category",
	"mainpage-nstab": "Main page",
	"nosuchaction": "No such action",

https://github.com/wikimedia/mediawiki/blob/REL1_26/languages/i18n/de.json#L368

	"nstab-category": "Kategorie",
	"nosuchaction": "Diese Aktion gibt es nicht",

There is no translation for mainpage-nstab it seems.

I did ask about getting REL1_26 exports done before we actually made a release.

The general answer here was a lack of tooling for tracking eligible translations (ie what hadn't been changed in english, or if they had, no parameter addition/removals)

In T119938#1855329, @Krinkle wrote:

Looks like that translation is not exported for 1.26. Maybe it was translated after the branch cut?

So is that something that someone (who?) could/should still fix, or what's going to happen with this task? :)

In T119938#2069732, @Aklapper wrote:

In T119938#1855329, @Krinkle wrote:

Looks like that translation is not exported for 1.26. Maybe it was translated after the branch cut?

So is that something that someone (who?) could/should still fix, or what's going to happen with this task? :)

Yes. The message in question (and various others) are essentially English-only, even in the current REL1_26 branch state.

translatewiki.net should do an export to REL1_26 and we should put it out there in a minor release as quickly as possible.

Arguably depends on T90169, but someone wanted to write a script to replace the (semi)manual steps Siebrand used to make for the l10n backports.

AFAIK, Language-Team never took care of MediaWiki i18n exports or infrastructure or MediaWiki releasing before. If you are requesting them to do so, it's probably better to directly write to their manager, as your tagging may be misinterpreted or not seen.