Out of the box localization capability of SitePublisher/LiveSite

System

I have a requirement for multilingual content on a site, potentially at the component level. The description of the out of the box capability for localizing based on a component that loads a DCR seems like it would do exactly what I want, but the docs are missing a crucial detail.

How does LiveSite determine the locale? Is it just looking at Accept-Language from the HTTP request, or is there a way to override that. A typical scenario might be a native Spanish speaker accessing the site from a public library terminal. I'd like to be able to shift to Spanish content either based on a user action (like clicking a link that says Espanol) or by virtue of them logging in and having Spanish as their language preference.

It isn't clear if there's a way to get that kind of magic out of the box, or if I would need to code that logic in an external (actually, probably better in a pre-controller since I'd want the functionality to just work for any FormsPublisher component).

Thoughts?