I note also that the Brazilian flag, rather than the Portuguese flag, is used for the Portuguese language.
The OP isn't suggesting that no English-language translation have the British flag, but that some have other flags, including the American flag, based on nation of origin. That's why the OP referred to the use of the Mexican flag. (I encounter Mexicans every day, and they simply call their language “Spanish”. Meanwhile, there are various differences between written American English and written British English. So let's have no more of trying to claim that it's somehow different for English from the cases for Spanish and for Portuguese.)
But all this gets messy. Many nations, including America, are multilingual. What does one do with a Spanish-language translation from a New Yorker of Puerto Rican descent? And what does one do with a translation produced in Japan, throwing together both Americanisms and things normally said or written with at least one pinkie extended (such as “whilst”)?