Catalonia is still considered a part of Spain so I don't think that's allowed for now. Maybe in the future when Catalonia declares independence and becomes recognized by the other nations as a sovereign country? Regarding Andorra and France.... well, you have to show proof that you are either Andorran or French, or I doubt the admins will even consider changing your flag.
Ireland actually has its own flag while Northern Ireland doesn't because it's a part of the United Kingdom. Same as Wales and Scotland. It's funny that both Isle of Man and Guernsey have their own flags though, but probably because they're crown dependencies and independently administered jurisdictions which means they aren't under the United Kingdom nor the British Overseas Territories.
Hope this cleared stuff, and I hope I did not come off as rude. I can understand Catalans having this sentiment but sadly I don't think peppy will consider this at all unless you show proof of citizenship.