Cannot update custom URL for flag images

Hi, I try to input a URL in the section of Custom URL for flag images, but the frontend cannot load the update flag image.

The default flag image is loaded according to the the site language.

I try to add other languages but set all the site language as English(UK).

The frontend will load the UK flag for all the languages.

I have checked the guideline, but it does not have detailed instructions.

https://msls.co/user-docs/plugin-configuration.html#advanced-settings

“Custom URL for flag images
There are flag-images included but you can set also a custom directory if you have your own set.”

Do you have more information about how to properly update custom URL for flag images?

Thank you!