If you delete a localization in project-info, the language actually still remains some how in the project. You run the app, the app still shows the untranslated keys. But if you run the app in a language that has not been add to the project before, then the app will show the English(default) locale values. Just don't know how to fix it, no any docs on the internet or apple's site.

And after hours search I still can't find a way to remove a language support.

I can't believe it. If this is the expected behavior, why it's in grey and showing the English text in string catalog, it's misleading. If a language, like spanish, is not provided, then the app will show English values(the default locale), or the keys? After test, it will show the keys, this means the locale fall back is not working, also means that the "default locale" is meaningless.

It's like "always on top" option to all apps.

Although Apple introduced .toolbar(removing: .sidebarToggle) finally, users can still drag the divider to change the column width and hide the side bar on macOS, and since sidebar toggle button is removed, users could never get back. In Sonoma Xcode15, developers have to use something like this: .introspect(.navigationSplitView, on: .macOS(.v13,.v14,.v15)) { splitview in if let delegate = splitview.delegate as? NSSplitViewController { delegate.splitViewItems.first?.canCollapse = false } } But it fails to work on Sequoia with Xcode16. Developers have to search around again to make it work just like the built-in Settings App, it's a shame.