Not specially related to 4.0
The code has been changed drastically since 2016.
Now we have a general string for any package
$msg = JText::sprintf('COM_INSTALLER_INSTALL_SUCCESS', JText::_('COM_INSTALLER_TYPE_TYPE_' . strtoupper($package['type'])));

in .../administrator/components/com_installer/models/install.php line 197

It uses COM_INSTALLER_TYPE_TYPE_PACKAGE="package" instead of COM_INSTALLER_TYPE_TYPE_LANGUAGE="language" because one can still (I guess. Would have to test) install a single language (site or admin) which would not be presented as a pack. (That kind is not proposed by core)

Example whne I Installed the Russian admin.zip via extension install

Either we need to have a new way of saying that a language pack has been installed (similar to the old way) or at a minimum the unused language string should be removed

Without some kind of special attribute or some way to identify a language pack within the package adapter you won’t get the desired end state anywhere that isn’t the languages install page. Because ultimately a language pack is just a collection of language extensions, and I don’t think it’s a good idea to try and assume something is a language pack based on file name or it only containing a certain extension type.

And, in any case, one can install (although not recommanded by core), a single language as an extension and not as a pack. Which means we should keep the string.

It is crazy that on a page that can only install a language there can not be a message that says you installed a language. Saying that its not possible is something I can not believe.

I guess I wasn't clear enough that I was referring to installing a language the recommended way using the language specific install page and not the generic extension installer

It was clear. Just explaining why we should keep the string. @mbabker explained the package issue.

git blame says 204ef46 undid that change. So it was added in 3.6 and removed in 3.7 without notice.

That PR was real good and this very small side effect is imho epiphenomenic.

When using big words its helpful to use the correct one.

Side effect or not - it should be fixed

Then fix it. The word is correct. Learning semantics can be very useful for computer code as well as daily life. ROTFLOL

I will fix it. You are just stubbornly refusing to consider anything that was not your idea or that you overlooked

Stop insulting people please.

Closing as we have a patch #26310