Extension names are translated in getHumanReadableLogMessage().

Well, as demonstrated above, they are not.
What do you suggest?

Language files are not loaded. Because your extension name is uppercase:
<name>PLG_CONTENT_JOSTAG</name>

Need to pass lowercase version to loadTranslationFiles() or change it to lowercase inside.

<name>com_localise</name> is lowercase.

It lacks extension name (COM_LOCALISE) string in main language file. Note loadTranslationFiles() doesn't load .sys file.

hmm

Perhaps we should load .sys files too. In case an extension doesn't come with main file (e.g. plugins without any params)?

We should cope for both issues imho. Uppercase and .sys.ini

Issue for uppercase can be easily solved by modifying loadTranslationFiles()

	public static function loadTranslationFiles($extension)
	{
		static $cache = array();
		$extension = strtolower($extension);

then we do not need any more to strtolower to load the lang file.

And also force load the sys.ini file in that same file.

I can do that or you do?

Just do it ✔️

not as simple as I thought for .sys as they may depend on the client of the extension.
I may just let this one alone.

This made me realise we have 2 other issues:

case 'mod':
				$source = JPATH_SITE . '/modules/' . $extension;
				break;

does not take into account admin modules.
and

			case 'tpl':
				$source = JPATH_BASE . '/templates/' . substr($extension, 4);
				break;

does not take into account admin templates.

This method really need some improvement. On it.

Forget modules and templates. It looks like they are using the title and not the extension name.
Therefore loading their ini files is useless.
Will not touch it. Only the strtolower

Please see #22637