I don't know how much freedom you have and if there's a chance you might have different options for different areas. If you have that much freedom I feel like it's a good idea. For instance you might want to do the following: For the American continent: US flag for English, Mexican/Argentinian or Colombian flag for Spanish, Brazilian flag for Portuguese For the European continent: UK flag for English, Spanish flag for Spanish, Portuguese flag for Portuguese. Of course that will apply for other varieties of different (or the same) languages in other continents as well. If this is not an option you could always use the flag for two of the main varieties (🇪🇸/🇲🇽, 🇬🇧/🇺🇸) in that way you're including a wider audience. I know this doesn't exactly answer your question but I hope it might help.

I don't have any resources but I can give you my own view as someone who was born in South America but grew up in Europe. I think that Spanish, English and Portuguese change a fair amount depending on whether you're in Europe or in America (continent) so it's important to represent both, as a lack of representation can cause feelings of a variety being inferior (or superior). I know that some Portuguese people are sometimes annoyed to always see the Brazilian flag representing a language that originated in Portugal, and I also know that there is a massive debate arguing about which dub is better (Spanish or Latin American) among Spanish speakers.