[nightfeather]

Actually there is a header for this purpose [Accept-Language on MDN](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Ac...).

But some sites just don't care about it and try detect this base on other information.

edit: formatting

[dskloet]

I believe Google (used to?) ignores it if it's set to only English because that's too often the default. If that's the case, the work around is to set a second preferred language.