This breaks install of a clean 4.0 here:

@roland-d
I solved the issue by moving

		// Ensure we load the namespace loader
		$this->createExtensionNamespaceMap();

to the doExecute() method.
i.e.

	/**
	 * Method to run the Web application routines.
	 *
	 * @return  void
	 *
	 * @since   3.1
	 */
	protected function doExecute()
	{
		// Ensure we load the namespace loader
		$this->createExtensionNamespaceMap();

		// Initialise the application.
		$this->initialiseApp();

		// Dispatch the application.
		$this->dispatch();
	}

as done in
/build/stubGenerator.php

@infograf768 When do you get this? When I open the Joomla site I get this:

This is the URL /installation/index.php

I got the same error.
Download your branch.
Run composer install and npm ci.
Proceed to install in browser:
http://localhost/joomla-cms-feature-language-installation-error/installation/index.php

@Quy, I followed your steps and still get the normal installation screen.

The reason I put it in the InstallationApplication is because in the main CMS this code is also located in the CMSApplication and ConsoleApplication. So the InstallationApplication looks like the logical place to put it.

I would like to know what the actual error is. The message shown now is just a user message and not the actual error itself.

Later I will test it on another machine and see what happens there.

It is OK to place it in InstallationApplication imho.
It is the placement of $this->createExtensionNamespaceMap(); which has solved the issue here.
Can you test with my proposal?

@infograf768 Putting the code in doExecute() also works for me. I just really wonder why it in the __construct() works for me.

Let's keep it in doExecute if it works for the majority :)

I have tested this item ✅ successfully on 58fff76

I have tested this item ✅ successfully on 58fff76

@Quy
Can you please test again so that we get at last rid of this issue?

I have tested this item ✅ successfully on 22482d2

RTC

RTC

Thanks!