When I try to access my Moodle 4.1 site but the database is not available, I get the following error which is visible if debugging is set to display error in DEVELOPER mode:

Notice: Undefined property: stdClass::$lang in lib/weblib.php on line 2239.

Steps to reproduce:

1. Ensure that debugging is enabled by default in php.ini (E_ALL)
2. Complete the install Moodle 4.1 in English (en).
3. Stop the database server.
4. Go to the home page of the site.

Screenshot