I would add a note about the keys. Something like this:

If you want to use whole phrases instead of short keys, be careful - each dot is considered an array separator. You will not be able to get the correct translation.

return [
    'Done. The email has been sent successfully.' => 'Done. The email has been sent successfully.'
];

@neznaika0 Added a note. 7e2ea39
and see the test code: 8319a3c

Added about missing Language locale: 3cb3839
See https://forum.codeigniter.com/showthread.php?tid=76017

Good. I changing the language in the request (in Filters). It's the same solution.
$request->setLocale($settings->get('language'));