they can do that, actually they are doing it (read bellow)
let's think we have 2 3ds in spanish one eur, one usa, and both have ZeldaOot
if you play ZeldaOoT USA in the EUR console it will boot in english, and if you put the Oot EUR in the USA console it will boot in english, but if you match the region USA-USA EUR-EUR it will boot in spanish (check it out in CFW). But if you use GW both boot in spanish, so they know how to do the language switch