The language units offered in the Intensive terms are only open to students doing the accelerated stream of a Diploma of Language Studies. It allows students who do not have enough room in their degree program to do what is the equivalent of a language major outside of their degree. Plus you get an extra qualification from it.
If you are interested in learning French, then I would definitely recommend enrolling in a DipLang; you can do it concurrently with your regular degree, and the majority of the work is done in the term breaks, so there is minimal impact on your other studies. You can apply through the website here:
http://sydney.edu.au/courses/diploma-of-language-studies.