Fucking do it.
I made the mistake of deciding to take one up at the end of first year.
Basically a DipLang works by adding 100 points of study to your degree. That's the equivalent of one year. Normally, your science degree has 300 points of study. Unsurprisingly, that's three years.
Every semester, you have four slots which can be filled with subjects. If you are only taking a science degree, then you only fill those 4 subjects with Science subjects. Note that normally one of those is a breadth subject every semester. If you pick a conventional degree, then every subject is worth 12.5 credit points, meaning your overall semester is worth 50; and a year is worth 100.
With a Diploma of Languages, all you are doing is adding on an extra 100 points. Or an extra year.
The thing is that you then have to put in all these extra slots into the same amount of slots (four slots per semester). That means that for every DipLang subject you put into those 4 slots, you have to take one science subject (or breadth subject) out.
It might sound like you are really complicating your degree if you do that, but trust me - it makes everything a lot simpler when compared to what you need to do if you apply at the end of first year as I've done - my degree is a little bit of a headfuck at the moment.
Now don't think of taking on a DipLang as extra concurrent study time devoted to the normal degree, because you aren't going to overload every semester of uni to get it fast tracked (if you do 5 subjects per semester, then you knock off 62.5 credit points instead of the usual 50 every semester, which means you reach the destination 400 faster). That's virtually impossible because you need to maintain a solid GPA and not fail any subjects in addition to a bunch of other criteria in order to successfully overload.
Think instead that the DipLang simply adds a year onto the degree, because you can't study more at any one given time - so you have to spread the things you're choosing to study out over a longer period of time.
I personally believe it's highly advantageous for a couple of reasons.
1. You spend longer at uni
2. Languages are GPA boosters (at least for me, they are), and
3. You open up prospects of bilingual or even trilingual PhDs.
Now to elaborate:
1. You spend longer at university - this is great, because I love uni. I don't know about you, but I am absolutely loving the uni student life. I moved out and became fully independent thanks to tutoring and then basically live a typical broke student uni life - constantly going out whenever I'm not studying or working. This is a lifestyle that I absolutely love, and I'd hate to be a working professional simply because you can't go out on a Tuesday night. At uni, your timetable is fucked, but it's so variable, which means your get weekdays off if you play your cards right. There's so many perks of being a uni student - you can sometimes qualify for austudy/youth allowance etcetera. If you're choosing to be at uni and you don't have your mind set on a concrete career, then the more time you have to think things over, reflect, and plan for the future - the better. The only time I can see this not being great is if you don't enjoy university (you can just drop the Diploma and get out of there more quickly) or if you want to definitely become, say, a doctor in the minimum amount of study.
2. Languages at uni are stupidly easy. Remember how in VCE they were the subject to pick if you wanted scaling? Yeah, well, not anymore. Their difficulty pales in comparison to the content that you have to learn in even your first year science subjects. I learned more bio words in a year than I have Russian ones in my language subjects. I guess it depends on the mindset you have. I hardly had to put in any work into my Language subjects. In fact, I didn't. All I did was rock up to classes and learn the bare minimum for assignments (2hrs per week on the assignment was all I ever did, when I had to do much more for chem and bio). And I still got very good marks that I was very happy with - my highest mark ever in fact was Russian 2 with an 82 H1. I was elated. I love the fact that I can just pick languages and chill out with my degree, taking less science subjects - which are incredibly hard - so I can devote more of my time to fewer of them, and then still have spare time to learn langauges.
Note that this also plays well into my hands because my diploma extends the time that I'm at uni - meaning even MORE spread out language subjects.
3. If you get into and do well in an Honours year, then you're really opening doors for yourself if you pick a language which has good PhD funding. Maybe not so much Russian, is what I'm saying - pick something that's either good for research (i.e. German) or something a little more global with a lot of speakers in a lot of different countries (i.e. Spanish).