The issue is present on LINUX systems.

Thanks JM!

In the Windows platform, one cannot encounter thie issue as the filenames are handled in case insensitive manner.

In the Linux platform, where the filenames are case sensitive, one would encounter this issue.

Please see the images tested on a Linux platform.

The testing was done with both the languages Chinese, Traditional (zh-TW) and Chinese, Simplified (zh-CN). The results are the same. For illustrative purpose, only the result of the language zh-TW is shown.

To test this, the Default Administrative Language has to be changed to changed to appropriate language. In this case it would be Chinese, Simplified (zh-CN).

Image 1: Before renaming the file zh-CN.js

Image 2: After renaming the file zh-CN.js to zh-cn.js

Code concerned is here
https://github.com/joomla/joomla-cms/blob/staging/libraries/joomla/form/fields/calendar.php#L283-L290

@infograf768 for J4 I will automate this the same way I did for the installation error page, eg using the node build.js tools: https://github.com/joomla/joomla-cms/blob/4.0-dev/build/build-modules-js/installation.js. This means the files will get generated automatically through the ini files, so we rule out such bugs...
For J3 we need to somehow patch it

I have asked Mig to test with this patch

diff --git a/libraries/joomla/form/fields/calendar.php b/libraries/joomla/form/fields/calendar.php
index 8ffb6d9..1987143 100644
--- a/libraries/joomla/form/fields/calendar.php
+++ b/libraries/joomla/form/fields/calendar.php
@@ -285,4 +285,8 @@
 			$localesPath = 'system/fields/calendar-locales/' . strtolower($tag) . '.js';
 		}
+		elseif (is_file(JPATH_ROOT . '/media/system/js/fields/calendar-locales/' . $tag . '.js'))
+		{
+			$localesPath = 'system/fields/calendar-locales/' . $tag . '.js';
+		}
 		elseif (is_file(JPATH_ROOT . '/media/system/js/fields/calendar-locales/' . strtolower(substr($tag, 0, -3)) . '.js'))
 		{
diff --git a/libraries/src/HTML/HTMLHelper.php b/libraries/src/HTML/HTMLHelper.php
index b13fe52..7b0ece4 100644
--- a/libraries/src/HTML/HTMLHelper.php
+++ b/libraries/src/HTML/HTMLHelper.php
@@ -1014,4 +1014,8 @@
 			$localesPath = 'system/fields/calendar-locales/' . strtolower($tag) . '.js';
 		}
+		elseif (is_file(JPATH_ROOT . '/media/system/js/fields/calendar-locales/' . $tag . '.js'))
+		{
+			$localesPath = 'system/fields/calendar-locales/' . $tag . '.js';
+		}
 		elseif (is_file(JPATH_ROOT . '/media/system/js/fields/calendar-locales/' . strtolower(substr($tag, 0, -3)) . '.js'))
 		{

Let's see if it works

I am told it works. Making new PR

replacing by #19662