Just create [[MediaWiki:Mainpage-nstab]]

Do you want a default translation for the optional message and make it non-optional?

Well I think the tab should be for main page by default it should say Main Page and allow people to opt out by doing what you suggested.

We could mark it with translatewiki [{{Identical|Main page}}](https://translatewiki.net/wiki/Template:Identical/Main_page) as well. I'd recommend against making it default to {{int:mainpage}} since some wikis have a namespaced and/or camel-cased title there that isn't a good label (e.g. Special:MainPage or Project:Main_Page).

Message {{int:mainpage-description}} (as used in the sidebar) would be more appropriate in that case. This way wikis that override it with a different page will still have the correct (and localised) label.

I agree it makes sense to do this by default instead of requiring wikis to do it manually.

Ok yes. Where should I change and which message do I change.

@Paladox In MediaWiki core, set key mainpage-nstab in messages en.json and qqq.json.

Thanks.

Change 239698 had a related patch set uploaded (by Paladox):
By default have Main Page test on tab

https://gerrit.wikimedia.org/r/239698

Change 239698 merged by jenkins-bot:
By default have Main page text on Main page tab

https://gerrit.wikimedia.org/r/239698