Child of Atum template does not show correct text at login page

[pierre-pvln]

Steps to reproduce the issue

Create a child template for Atum template
Added a user.css in css folder of atum template (/media/templates/administrator/atum_child/css/user.css)
Set this child as the default template

Expected result

in login page sidebar left:
Joomla Administrator Login

Actual result

TPL_ATUM_BACKEND_LOGIN

System information (as much as possible)

Joomla 4.2.0
PHP 8.0

Additional comments

When I switch back to atum default (the parent) the correct text is shown.
I have no user.css in atum default.

[brianteeman]

Confirmed

[chmst]

The system tries to load the language file tpl_atum_[childname] from the language folder, which does not exist. Probably the same in frontend.

[brianteeman]

Its not just the login page it is all the pages that have this problem. It's not noticable visually on the template dashboard for example but it certainly is for a screenreader where it breaks a lot of strings

[alikon]

please follow the discussion of this issue here #38568