If you're placed in Japanese 5 it means you complete Japanese 5 in Semester 1.
Provided you pass, you would then go on to study Japanese 6 in Semester 2 of your first year.
Second year would be Japanese 7 and 8, third year Variations in Japanese and Japanese Literature.
This sequence is the same as a major, minor, breadth or diploma.
The difference is that for major, minor and diploma you get an award (/degree) and you are required to complete more than just those language units.
As a breadth, you only need to go through the language sequence. You won't get an award though. But you do have the freedom to do Japanese, other subjects, whichever you choose really. You could do Japanese 5 and then idk a music subject for semester two if you wanted. The is generally one subject as breadth per semester.
The diploma adds one year to your degree provided you do not fail units and you meet the subject requirements. It works by taking some of your elective and breadth subjects from your bachelor and contributes them towards the unit requirements for the diploma. Then you complete extra due the diploma in your fourth year (and any failed units or whatever to achieve the bachelor).
Essentially, some of the subjects in your bachelor, which would count 1 unit or 12.5 credits towards your bachelor, also are contributing towards the credit requirements in the diploma. Two for the price of one.
If you're looking into Japanese for a reason other than idk fun or just to learn, then probably a diploma is more useful? I'd beg to differ though. If you're interested in interpreting or translation then doing a Diploma of Translation at RMIT or something after graduating is more worth your time. Even teaching, you could spend that extra year getting a dip Ed.
But if it's for a hobby I recommend it as a breadth. Or not even maybe. I find Japanese is more fun when it isn't in a classroom. But to each their own!
Good luck