The answer marked as "correct" is explicitly wrong.
ENG doesn't mean "England," it means "English"
The tag is referring to the LANGUAGE of the game it came from, not the real-world region. If you play the game in a language from another country while your GPS and 3DS settings indicate you're somewhere else, you still get the tag, because it's not about location. It just so happens most people pick their native language, and live in a place where their native language is the most common one.
American-born Pokémon have "ENG" when traded to other language games, and Canadian-born Pokémon have either ENG or FRE when traded to other games, depending on if the Canadian playing is French-speaking or not. Mexico-born Pokémon tend to have "SPA" too.