I think there's just a lot of confusion as people are discussing different things here.
I know that some models won't let you change language at all if within a specific region. Japan is the one I'm mostly familiar with - Japanase routers are hardcoded to show the webui in Japanese. That might also be the case for other regions: this, I'm not 100% sure.
However, some of the requests here have been about the initial router setup pages, rather than the regular configuration interface you can access afterward to manage your router.