customize flags

I tried Following the faq in order to change the flag pictures. My own flags won’t appear.
1. put flag nl_NL.png and en_UK in folder http://www.example.com/wp-content/polylang/
2. emptied cache
3. nothing changes

Could someone provide some instructions that are suited to an advanced developer, because these are not sufficient:

Can I use my own flags for the language switcher?

Yes. You have to use PNG or JPG files and name them with the WordPress locale. For example, en_US.png. Then upload these files in the /wp-content/polylang/ directory (create the directory if it does not exist). Don’t use the /polylang/flags/ directory as your files would be removed when automatically updating the plugin. Once the custom flags are uploaded, go in Polylang settings and click on save changes. Note that your custom flags won’t be used on admin side.

https://polylang.wordpress.com/documentation/frequently-asked-questions/the-language-switcher/

https://ww.wp.xz.cn/plugins/polylang/