Regarding flagicon, that wiki has uploaded a thumbnail of all flags;

You should copy the data at https://marvelcinematicuniverse.fandom.com/wiki/Template:Citizenship and I think you should download all of the flags in that page, and then you can upload them in your wiki via https://dev.fandom.com/wiki/UploadMultipleFiles.

Edit: Most of the flags are able from the script https://dev.fandom.com/wiki/Flags

@Merlin the Immortal The problem with the infobox is that when I make changes, such as add a new field or add new information, it doesn't show up when I save it.

Could you provide a link to the page that has the infobox?

And also do you try saving the page or just apply (save) the infobox information in Visual Editor and when you see it's not inserted then you don't save the page?

You can't just add parameters that easily; Read about infoboxes at https://community.fandom.com/wiki/Help:Infoboxes

And do know that you can only use parameters which are created in https://lady-justice.fandom.com/wiki/Template:Character?action=edit

For instance there's no parameter called "Citizenship".

And don't forget that parameters are case-sensitive and "image" parameters must get an entry in the format below:

[[File:Example.png]]
or
<gallery>
Example.png|example
Example2.jpg|example
</gallery>