How how to add flags?

[Evgeny]

How to add flags of other countries to the existing list instead of letters?

[Angel_Crawford_ftw]

Windows don't want to support pretty flag Emojis anymore and instead uses the two-letter country codes only. That's what you are seeing in the image.
Chrome only supports those two-letter country codes, too. Last time I checked, Firefox supported pretty flags, but could be an old information.

I can't say for sure why those pretty flags aren't supported anymore, but it could be the that in the past some flags where heavy discussed because of "Too old", "Needs a renewal", "Not supported anymore", "Where is our flag", "We don't use this flag"... and so on.
But the biggest fact could be, that some two-letter country codes uses more then one country flag. So, which flag should then be used? Fastes way: Don't show them anymore

Solution:
Search for nice flags in image form in the web and use the "Select file..." Button to show nice looking images.

Edit: Added solution

[Evgeny]

Thanks for the quick response!
Where are situated these icons of flags? Can I change some of them for more actual?