Changing the request processor to know about locales was exactly the approach I took with Greenpeace Planet.  In GP's case things are a bit more complex because we map to both a NRO (National/Regional organization - i.e. GP Nederlands vs. GP Deutchland) and language (think GP Canada/French vs. GP Canada/English.)

The legal mappings are all nsv-cached at start-up and kept consistent by the admin UI that manages the mappings, so there's no database hit.  We also use cookies to remember preferences for the user between sessions, if they have cookies enabled in their browser.

If you guys want to look at the code you're more than welcome to, of course, the GP code is GPL'd.