There are currently a lot of requests from users missing the flag for their territory (Taiwan, Palestine, Kiribati, Faroe Islands, etc).
I can see that Geoguessr already uses ISO 3166-1 alpha-2 code internally to store the flag for the user.
However, Geoguessr have 194 options, while the full ISO 3166-1 alpha-2 list contains 242 officially assigned codes.
So my suggestion is to use the full list, and not an arbitrarly truncated list.
For context, ISO 3166-1 alpha-2 codes are two-letter country codes, published by the International Organization for Standardization (ISO), to represent countries, dependent territories, and special areas of geographical interest.
Geoguessr players know these codes very well, because besides a few exceptions, that's what is used for Internet's country code top-level domains (example: www.faroeislands.fo ends with fo which is the country code for Faroe Islands).
This is a widely recognized reference, and by following it, you won't get into political trouble about the definition of "country", since you're following a standard.
Thank you in advance for considering this suggestion to keep making Geoguessr a welcoming platform regardless of your origin.