Can you check the browser's dev console to see if the network request is having any errors or if there's a JavaScript related issue?

Sure. I added the screenshot to the initial post.

Just an addition. I checked the server return. My server had the error level pretty high so I see some notices which break the JSON response:

Notice: Undefined property: stdClass::$min_dev_level in /var/www/eventgallery/tests/joomla-cms3/libraries/src/Updater/Update.php on line 326
Call Stack
#	Time	Memory	Function	Location
1	0.2007	385760	{main}( )	.../index.php:0
2	0.2086	1077992	InstallationApplicationWeb->execute( )	.../index.php:32
3	0.2086	1077992	InstallationApplicationWeb->doExecute( )	.../CMSApplication.php:267
4	0.2092	1097856	InstallationApplicationWeb->dispatch( )	.../web.php:200
5	0.2118	1266008	InstallationControllerInstallLanguages->execute( )	.../web.php:161
6	0.2144	1278080	InstallationModelLanguages->install( )	.../languages.php:61
7	0.2196	1436200	InstallationModelLanguages->getPackageUrl( )	.../languages.php:164
8	0.2227	1441848	Joomla\CMS\Updater\Update->loadFromXml( )	.../languages.php:237
9	0.7021	1448488	xml_parse ( )	.../Update.php:483
10	0.7025	1453656	Joomla\CMS\Updater\Update->_endElement( )	.../Update.php:483

( ! ) Notice: Undefined property: stdClass::$max_dev_level in /var/www/eventgallery/tests/joomla-cms3/libraries/src/Updater/Update.php on line 327
Call Stack
#	Time	Memory	Function	Location
1	0.2007	385760	{main}( )	.../index.php:0
2	0.2086	1077992	InstallationApplicationWeb->execute( )	.../index.php:32
3	0.2086	1077992	InstallationApplicationWeb->doExecute( )	.../CMSApplication.php:267
4	0.2092	1097856	InstallationApplicationWeb->dispatch( )	.../web.php:200
5	0.2118	1266008	InstallationControllerInstallLanguages->execute( )	.../web.php:161
6	0.2144	1278080	InstallationModelLanguages->install( )	.../languages.php:61
7	0.2196	1436200	InstallationModelLanguages->getPackageUrl( )	.../languages.php:164
8	0.2227	1441848	Joomla\CMS\Updater\Update->loadFromXml( )	.../languages.php:237
9	0.7021	1448488	xml_parse ( )	.../Update.php:483
10	0.7025	1453656	Joomla\CMS\Updater\Update->_endElement( )	.../Update.php:483

Seems like this code is causing the notice messages:

	$patchMinimumSupported = $this->get('jversion.dev_level', Version::PATCH_VERSION) >= $this->currentUpdate->targetplatform->min_dev_level;
				$patchMaximumSupported = $this->get('jversion.dev_level', Version::PATCH_VERSION) <= $this->currentUpdate->targetplatform->max_dev_level;

```<hr /><sub>This comment was created with the <a href="https://github.com/joomla/jissues">J!Tracker Application</a> at <a href="https://issues.joomla.org/tracker/joomla-cms/17452">issues.joomla.org/tracker/joomla-cms/17452</a>.</sub>

@mbabker I guess this commit causes the issue: wilsonge/joomla-cms@e130087

Hope this helps. Seems like this is actually unrelated to the multilanguage category.

Ya, the isset checks from the block below the $patchMinimumSupported variable needs to be added to that line (same for the max).

I can confirm that there are PHP notices shown on a 3.7.4 updated to 3.8.0-beta in the Joomla Update view on "Undefined property: stdClass::$min_dev_level" and other stuff related to the change of the version number variables. So there seems to be something wrong with that on updated systems but as it looks like it does not happen on clean staging installation.

The old constants are still present so that in and of itself is not an issue.

The issue is when processing update XML files that do not have min_dev_level and max_dev_level attributes on the targetplatform tag, the isset check needs to be moved up to be included in the $patch* variables.

$patchMinimumSupported = !isset($this->currentUpdate->targetplatform->min_dev_level) || Version::PATCH_VERSION >= $this->currentUpdate->targetplatform->min_dev_level

Just found that too in 3.8.0 beta
#17462
It happens on clean staging install here.

Based on @mbabker suggestion, I tested this patch and do not get errors anymore:

diff --git a/libraries/src/Updater/Adapter/ExtensionAdapter.php b/libraries/src/Updater/Adapter/ExtensionAdapter.php
index 63e3043..b6d645e 100644
--- a/libraries/src/Updater/Adapter/ExtensionAdapter.php
+++ b/libraries/src/Updater/Adapter/ExtensionAdapter.php
@@ -125,11 +125,11 @@
 				 * Check for optional min_dev_level and max_dev_level attributes to further specify targetplatform (e.g., 3.0.1)
 				 */
-				$patchMinimumSupported = Version::PATCH_VERSION >= $this->currentUpdate->targetplatform->min_dev_level;
-				$patchMaximumSupported = Version::PATCH_VERSION <= $this->currentUpdate->targetplatform->max_dev_level;
+				$patchMinimumSupported = !isset($this->currentUpdate->targetplatform->min_dev_level) || Version::PATCH_VERSION >= $this->currentUpdate->targetplatform->min_dev_level;
+				$patchMaximumSupported = !isset($this->currentUpdate->targetplatform->max_dev_level) || Version::PATCH_VERSION >= $this->currentUpdate->targetplatform->max_dev_level;
 
 				if ($product == $this->currentUpdate->targetplatform['NAME']
 					&& preg_match('/^' . $this->currentUpdate->targetplatform['VERSION'] . '/', JVERSION)
-					&& ((!isset($this->currentUpdate->targetplatform->min_dev_level)) || $patchMinimumSupported)
-					&& ((!isset($this->currentUpdate->targetplatform->max_dev_level)) || $patchMaximumSupported))
+					&& $patchMinimumSupported
+					&& $patchMaximumSupported)
 				{
 					// Check if PHP version supported via <php_minimum> tag, assume true if tag isn't present
diff --git a/libraries/src/Updater/Update.php b/libraries/src/Updater/Update.php
index 5a3dfb0..d342f84 100644
--- a/libraries/src/Updater/Update.php
+++ b/libraries/src/Updater/Update.php
@@ -324,12 +324,12 @@
 				 * Check for optional min_dev_level and max_dev_level attributes to further specify targetplatform (e.g., 3.0.1)
 				 */
-				$patchMinimumSupported = $this->get('jversion.dev_level', Version::PATCH_VERSION) >= $this->currentUpdate->targetplatform->min_dev_level;
-				$patchMaximumSupported = $this->get('jversion.dev_level', Version::PATCH_VERSION) <= $this->currentUpdate->targetplatform->max_dev_level;
+				$patchMinimumSupported = !isset($this->currentUpdate->targetplatform->min_dev_level) || Version::PATCH_VERSION >= $this->currentUpdate->targetplatform->min_dev_level;
+				$patchMaximumSupported = !isset($this->currentUpdate->targetplatform->max_dev_level) || Version::PATCH_VERSION >= $this->currentUpdate->targetplatform->max_dev_level;
 
 				if (isset($this->currentUpdate->targetplatform->name)
 					&& $product == $this->currentUpdate->targetplatform->name
 					&& preg_match('/^' . $this->currentUpdate->targetplatform->version . '/', $this->get('jversion.full', JVERSION))
-					&& ((!isset($this->currentUpdate->targetplatform->min_dev_level)) || $patchMinimumSupported)
-					&& ((!isset($this->currentUpdate->targetplatform->max_dev_level)) || $patchMaximumSupported))
+					&& $patchMinimumSupported
+					&& $patchMaximumSupported)
 				{
 					$phpMatch = false;

@mbabker
Can you make the patch or shall I (if this above looks OK to you) ?

Would you do the patch please?

will do tmorrow

Just ping me when the PR is ready so I can help with testing.

@svenbluege @AlexRed @richard67

Please test patch #17481