This fixes the too few arguments error: 8744332. But installing that package is giving another error which I'm still working on fixing.

Indeed, we now have
Call to undefined method JInstallerAdapterLanguage::set()

Note: Discover is also broken (for languages at least).

@wilsonge
Tested changing from

		if (!file_exists($this->parent->getPath('extension_site')))
		{
			if (!$created = JFolder::create($this->parent->getPath('extension_site')))
			{
				$this->parent
					->abort(
					JText::sprintf(
						'JLIB_INSTALLER_ABORT',
						JText::sprintf('JLIB_INSTALLER_ERROR_CREATE_FOLDER_FAILED', $this->parent->getPath('extension_site'))
					)
				);

				return false;
			}
		}

to

		if (!file_exists($this->parent->getPath('extension_site')))
		{
			if (!$created = JFolder::create($this->parent->getPath('extension_site')))
			{
				throw new RuntimeException(
					JText::sprintf(
						'JLIB_INSTALLER_ABORT',
						JText::sprintf('JLIB_INSTALLER_ERROR_CREATE_FOLDER_FAILED', $this->parent->getPath('extension_site'))
					)
				);

				return false;
			}
		}

And I do get this:

OK, I get it to install by adding the missing

$row->set('custom_data', '');
$row->set('system_data', '');

Still remaining the asset_id for the content language which could not be created.

Uninstalling a language (installed after modifying the adapter as posted above) gives

Too few arguments to function PlgExtensionJoomla::onExtensionAfterUninstall(), 2 passed in /Applications/MAMP/htdocs/joomla_400/libraries/cms/plugin/plugin.php on line 322 and exactly 3 expected

Hmm I don't see why onExtensionAfterUninstall would only be getting two params https://github.com/joomla/joomla-cms/blob/4.0-dev/libraries/cms/installer/installer.php#L754 This passes 3 in. I'll look into this in a little bit!

It looks anyhow (I let you make the patch with what I already proposed above) that the entire installer needs love and care: Install, Uninstall, Update, Discover. All fails on one type of extension or the other.

@mbabker can you just check JM's proposed patch for the set method. If abort is failing seems like there might be a deeper problem (that throwing the exception just covers up)

There probably will be. There's been some tidying up done of the installer code AND we rebuilt the plugin system. There's going to be lots of bugs. Just keep reporting them and I'll keep fixing them :)

I've fixed the plugin event. @csthomas can you look into the database queries here. there's a few places that is affected by custom_data and system_data (such as in #14409)

@infograf768 - Just tested installing the language pack from the link you provided and it seems to be working now.

Ok to close?

They install OK indeed. Will test again uninstall, etc.

They uninstall OK here too.

This may need to be re-opened. You can install the zip via the extension manager, but can't via the langage manager.

Unable to find install package

You mean through the Install Languages Manager.
That is normal.
The update site credentials are aimed at 3.x maxi and will not present any packs for a 4.x version.
We will have to create a new xml for that as well as a new repository for 4.x packs.

Ah fair enough. Thanks for clarifying