Searching for plugin: Multilingual multisite with monolingual sites

Hi, I’m trying to find a plugin for a multilingual multisite with monolingual blogs, similar to what I think WordPress.com is doing, but all I can find are plugins for making what are essentially translations of a single blog. Here’s what I’m looking for:

• Each site is monolingual: it has an individual locale (which typically would match the language in which the posts are written; this is the locale that would be shown to an unregistered guest) and is never machine-translated or anything.
• The sites are independent: there’s no “the same, but in Spanish” link because they won’t have translations of each other.
• Registered users can choose to override the sites’ default locale with their own if they wish so. Again, this would only affect the WordPress and theme messages and wouldn’t machine-translate the posts.

This actually seems something fairly simple, and I might take a shot at writing the plugin myself, if it wasn’t because I’m absolutely sure it must exist already. But I’ve had no luck searching. Could anyone point me in the right direction?