For official documentation of Codeigniter you can follow https://codeigniter4.github.io/userguide/outgoing/localization.html.
Please follow the below steps to add a new language.

1. Set default language from the app/Config/App.php.
   $defaultLocale= 'en'; and in supported language $supportedLocales = ['en', 'es', 'de', 'it', 'ru'];
   and set public $baseURL = 'https://yourdomain.com/';
2. All the files of each language are located in app/Language. You can create a new language folder by referring to the English language.
3. You need to add a new language in the topbar dropdown in the file app/Views/partials/topbar.php and app/Views/partials/horizontal.php file.
4. It is managed by the backend in the file app/Controllers/Language.php.
   You can use <?= lang('Files.Search') ?> to translate "Search" keyword in supported languages.