I've worked on a couple multi-language/location projects... Usually using the Accept-Language header as the default response to closest match or the default language (en-US fallback). In which case, I would allow for a cookie and/or url override that would set a cookie and redirect.

    https://en-gb.foo.com >> set cookie and redirect to bare domain
    ...?lang=en-GB >> set cookie and redirect without the lang portion
    <cookie>
    <Accept-Language Header>

I'm also a proponent of having a mini-flag for the current language as a dropdown/flyout menu in the upper-right of the header where each local/language has the appropriate flag/representation in addition to the language name IN that language.

You can use just language (en) instead of language+locale (en-US) as desired.

edit: also, make sure you set your cache directive to include the cookie/header in question. You can also work the other way and always use the locale for subdomain.