The reason for the setting not appearing until other languages are installed is a technical one, but for a first try, I think this fits the main use case. The main case is for sites where the admin is in a different language by default, having the option to switch back to English on a per-user basis is super useful.

Of course, the ability to choose any installed language is there, the only thing really missing from it is the ability to install a language that isn't already installed. And that is available in the general settings area, so it's not a hugely missing feature, really.

For me, this is a fantastic feature. It will help a lot since I'll be able to access our international sites using English without actually affecting any of the other users of those sites. Trying to clean up spam in the Hebrew forums without being able to read the interface was kind of tricky. :)