It would probably be easier for you to learn Spanish, given native English speaker and learning German. I know people who, when learning two similar languages at once, get confused and mix them up. If you think you won't run into this problem, then study Spanish.
I found Japanese 1 to be a lot of fun, if that counts for anything. It's also relatively easy, imo. German and English are very different to Japanese, which eliminates the "getting them mixed up" problem but makes studying Japanese more difficult in other ways.
Both languages have their merits, so studying either would be really valuable, even if it's only for a semester.
(predominantly because I've run out of first year units).
You should investigate your ability to take language units as level two or level three units. I know with Japanese, there are about five different unit codes for the same Japanese unit. They do this so people (like you) can take Japanese as second or third year electives, because they've run out of first year electives. I've done this - I'm enrolled in
level three Japanese 8 for sem 2 2012. If you click on the link, see how the course code is ATS
3148, making it a level three unit? Scroll down to the prohibitions for this unit. All of these prohibitions are also Japanese 8 - level one, level two, honors level and postgraduate level. Everyone takes this same class together and sits the same exam, it's just that some people are enrolled under different unit codes.
Edit:You can do Japanese 2 as a level two unit:
http://www.monash.edu.au/pubs/2011handbooks/units/ATS2142.htmlUnfortunately though, you cannot do Spanish 2 as a level two unit:
http://www.monash.edu.au/pubs/2012handbooks/units/ATS1192.htmlSo I guess if you want to be able to continue beyond one semester of study, you should study Japanese.