From confusion/not point of view, user should be seeing a single-language interface. If she chooses to use English (which can also be a default/fall-back choice), even if she is Swedish and has set browser settings thusly, English interface to all of the site's content should be provided.
I guess a tricky part maybe if content, as well as UI elements are available in multiple languages for a particular item, e.g. (purely hypothetical option) a bboard posting.
Otherwise content should be displayed in whatever language it is stored in, regardless of user preferences. HTTP headers should, however, be set in such a way that browser picks Unicode setting, to ensure that no 'bird signs' appear on a web page.
...just a few fractions of a ¢2
