Multisite Language Switcher – only US flag is displayed

I installed and set up Multisite Language Swicher. Awesome plugin. Thanks its developers!
I only have an issue with display of definite flags in the widget area in my theme.
Only EN flag is displayed (5 times) even though the links to localized sites are working properly.
I have RU, DE, FR and SE sites also.

The installed lanugaes in Wordress are English, Swedish and Serbian.

As I understood, I need to add more languages to WordPress – RU, DE and FR. And also add .mo and .po files and SE flag SE into Multisite Language Swicher?

Or maybe there’s an easier way of adding custom flags into widget?
(Tried custom URL option, but it requires a custom folder, not a custom file. The images returned 404 error).

Thanks in advance for your help!