Setting default language in config

Hello!

I'm using the latest CI version.
Currently, I'm trying to set up multi language support for my website.

In application/config/config.php the default $config['language'] is set to 'english'.
It's working as it should, and loads the application/language/english/english_lang.php file.

But when I change the $config['language'] to something else, for example, polish, it gives me an error:
Unable to load the requested language file: language/polish/english_lang.php

Why is it trying to load english_lang.php and not polish_lang.php from the application/language/polish folder?

Hope to get some input on this.
Best regards.