I have installed the en_us language pack. Under Language Settings, I have assigned it as the default language. I have not changed the Language autodetect (i.e. default: Yes).
The Frontpage indicates the language as English - United States (en_us). When I login in as Admin user, the language defaults to en_us. When I login as one of my test accounts, it changes to English (en).
I do not understand why the en_us setting is not being carried through to the "student" accounts.