Some visual comparision.
Current view:

Proposed view:

@Bakual maybe the install button on the toolbar together with the checkboxes should stay, as they do provide a way to install multiple languages in one go (this functionality is now removed).

As I wrote in the PR that isn't possible anymore with the current code. Since we don't have IDs anymore.
The install button now goes directly to the regular install package code and is handled by the "install" controller, not a specific language install anymore.
If it is deemed as an important feature, we could try passing the install URLs to a language specific controller task which then calls the installer one-by-one.
Or we could do it a bit more sexy and open a modal with a progress bar where the languages are installed using AJAX calls to the regular installer. But here I likely hit my JS knowledge boundaries ?

I'll give it a go tomorrow, it seems doable...

Hmm it looks like travis is complaning on cs?

Good idea of having an option to reinstall.

Q: Does the change from updates table to xml influence how creating a multilingual site on install would work? Dunno if it use another method, asking to make sure you still would be able to install several languages in one go, when this is removed when adding languages later here.

For now the Find Languages button is missing in your screenshot.

From PR description:

The other button "Find Languages" also isn't needed anymore as the list is taken directly from the XML file now.

Q: Does the change from updates table to xml influence how creating a multilingual site on install would work? Dunno if it use another method, asking to make sure you still would be able to install several languages in one go, when this is removed when adding languages later here.

The installation code is all contained in the /installation folder, so it should have no effect at all. To be sure I have tested it now and it passes through without any errors.
I saw that the native title isn't populated during CMS installation and also the content languages aren't created. However that also seems to be the case for staging. That would be an unrelated issue then. Can someone verify that? Not that I have a borked installation myself.

No impact on clean multingual install. One can install multiple language in one go as before. Good.

Native title (in the Installed Languages Manager) would not show until our packs site and admin xx-XX.xml do contain the new tag. This is a guess as I can't test really until we do have a pack with such native title is presented in the Install Languages manager.

Content Languages are created when installing a clean joomla with multilang.
Content languages are created when using the Install button with your PR.

But, if you delete a content language for a said language and then Reinstall the language, the Content Language is not created.

Native title (in the Installed Languages Manager) would not show until our packs site and admin xx-XX.xml do contain the new tag.

Makes sense, yes.

But, if you delete a content language for a said language and then Reinstall the language, the Content Language is not created.

That's actually expected since technically a reinstall is the same as an update (to the same version). And I think we're not creating content languages during update of a language pack.

During update, we do not need to create a content language.
When reinstalling, it is another situation: it may mean the specific language folders have been deleted (site and/or admin).

A reinstall and an update is the exact same thing for the installer. Both install a package over an existing installation. You don't have a way to distinguish it there.
So it's either also done during an update, or it's not done.
It would be an unrelated change anyway as the same behavior currently exists when you manually download the language pack and install it. It's the exact same code that is triggered then.

I have tested this item ✅ successfully on 3f6c84e

I have tested this item ✅ successfully on 3f6c84e

Test OK

RTC