What version of Joomla?

staging

I didnt get that

Unable to reproduce with staging. I didn't have to click Remove "installation" folder button twice like you did in your screencast.

tested again: Some result
New sub domain, downloaded staging again.

wamp 64x - chrome 60.0.3112.101

With IE i see no quotes, its working correct.

IE = failed = "installation" folder removed.
Firefox = failed = "installation" folder removed.
Chrome = failed = "installation" folder removed.

Still unable to reproduce using Chrome v60.0.3112.113 in XAMPP on Windows 10.

Please check that you select : English (United Kingdom)
i also made the mistake to select English us. Now all browsers are failing.

OK I can confirm the issue with English US.

Uk you mean?
English US = working for me

Sorry, UK is not working. US is working only because it has not been updated per this PR #17436 .

These strings should be modified to use escaped double quotes, or use single quotes

INSTL_COMPLETE_ERROR_FOLDER_DELETE="\"%s\" folder could not be deleted. Please manually delete the folder."
INSTL_COMPLETE_FOLDER_REMOVED="\"%s\" folder removed."

or

INSTL_COMPLETE_ERROR_FOLDER_DELETE="'%s' folder could not be deleted. Please manually delete the folder."
INSTL_COMPLETE_FOLDER_REMOVED="'%s' folder removed."

to get correctly

Another solution is to modify .../installation/controller/removefolder.php line 117-125 to use
htmlspecialchars_decode

// If an error was encountered return an error.
		if (!$return)
		{
			$app->sendJsonResponse(new Exception(htmlspecialchars_decode(JText::sprintf('INSTL_COMPLETE_ERROR_FOLDER_DELETE', 'installation')), 500));
		}

		// Create a response body.
		$r = new stdClass;
		$r->text = htmlspecialchars_decode(JText::sprintf('INSTL_COMPLETE_FOLDER_REMOVED', 'installation'));

@mbabker @Bakual
What do you prefer?

I would just use single quotes or escaped quotes.

i made a pr for it, so I'm closing this issue

PR is here #17718