From what I've gleaned from other q&a, it's possible to set up multiple localized sites under one installation of Craft, but can you assign separate administrators to each? I'd like to set up multiple sites under one domain for a company with several global offices, but each office needs to have control over only their site's content— and ideally some admins could have access to more than one. Possible?
Yes. you can:
Here I created a User group which has only access to the english part of the content. As you can see these are checkboxes. So you can grant access to multiple languages at once.