I can't enrol into the beginner Chinese subjects online, either, due to not satisfying the "non-study package requisite conditions." I assume this is because enrolment into Chinese is contingent upon the placement interview and once I've done that they'll manually enrol me in the appropriate stream.
At my advisor appointment I expressed interest in the Diploma of Languages and was told that they strongly advise students not to enrol in it until the end of the first year, presumably to ensure people enrolling in it first try the subject and are certain they like it and have the capacity to finish it. I was asked to instead enrol in my language as breadth, so right now my first year is 50% science and 50% breadth (the breadth comprising of the finance subjects I initially wanted to do as breadth -- FNCE10001, FNCE20001, as well as Chinese which I had intended for the DipLang -- CHIN10005, CHIN10006). Apparently, at the end of my first year, I can enrol in the DipLang and the language subjects I study as breadth will instead be credited to the diploma and that will free up the breadth slots again for use in later years. There is however a 37.5 point limit on level 1 breadth subjects, but fortunately one of the finance subjects I am doing is level 2 so I don't go over that limit, even with the language. If that would be a problem for you, though, I'd recommend straightening it out earlier, but I don't think the enrolment lapse date is until mid-February, so there's plenty of time.
Given that the two language subjects will eventually be credited to the DipLang and I get the breadth slots back, I don't really have a problem doing it this way, because the course structure should be exactly the same over the course of the four years as if I had enrolled in the DipLang immediately. I am not sure how this works with fast-tracking or any other degree other than the BSc, though.