The method that I use is:
1. Copy an existing civilization information, and rename it, and then paste it in civilizations_informations adding the data you want to put.
2. When you create a civilization a file with the data will appear, with a meaningless name Ex:11035jfkslclbahc, You will rename this file with the TAG you want but make it the same with the civilization information.
3. Since your civilization is registered you can change the flag to the one you want with this link https://image.online-convert.com/convert-to-png , remember that you need two sizes of pixels 68x44 and
Reza Shah reacted to a post in a topic:
Few question about flag-changing+ on custom nation