I just followed your new 2.5 tutorial on setting up a multi-language site. Good tutorial, and an enjoyable experience.
I'm trying to see how to leverage this for the backend. It seems the only way to get a different language in the backend is to manually set it when I log in. Even though I set the browser to German (and have German installed), the backend would not pick up these changes.
Also, I created a test user and assigned that user to a backend language of German, but when I login, I still see the default English.
[1] How do I get the "backend Language" for a user to take effect?
[2] Does setting the language on an admin module have any effect? So far in what testing I could do, it seems not.