I have several packs of juice (or multivitamin nectar, to be precise), and I can either keep them in our flat or put them outside, where it's freezing during the night. Is it going to do any harm to the juice if I do the latter?
Freezing juice will definitely not harm the juice itself, but it might harm your container if you're not careful. The juice can expand quite a bit during the freezing process, so make sure that your container isn't completely full before you freeze it.
If the juice is already in a container (i.e. you bought it) then keep in mind that the container might not be designed for freezing and could explode at freezing temperatures due to the expanding volume. If you're not sure, you might be better off transferring them to some large freezer bags.
I should also mention that freezing will only slow down the enzymatic activity that degrades fruit (and fruit juice) over time, not stop it completely. Most fruit juice has been boiled to inactivate the enzymes, but if yours hasn't, then you might notice (bad) changes in the colour and taste over time. If you plan to consume it within a few weeks, or if the juice already has various preservatives, then don't worry about it, but if it's very fresh juice and this is for long-term storage, then you might want to briefly boil it before storing if you're not sure about its history.
Keep in mind that the above will itself alter the taste, so if you plan to do it, test it on a small amount first to be sure that you can live with the "blanched" taste.
For the record, when taste is a big concern, some juices definitely suffer a big hit from freezing. Citrus fruit juices are probably among those that suffer most. So, if your multivitamin nectar is like some I've seen, then it contains partly orange juice and pineapple juice that would put it in this category.
Orange juice is concentrated before freezing. That's to preserve taste, although there's the auxiliary benefit that it's more compact. The history of developing this process goes back nearly 100 years and wasn't perfected until 1948. Before that, OJ was canned and probably tasted not great at all. (Though canning actually preserves vitamin C better.)
Additionally, for most foods, a slower freezing process is worse than a quick one. I doubt this effects nutritional value, but it could to some degree? The slower the freeze, the bigger the ice crystals get and the more they interfere with cellular structure. So, if you have to choose how to freeze something, choose the coldest place. Where I live, that's definitely in the freezer. I do this for steaks I'm storing for a month or so and want preserved as well as possible. But, hey, maybe in an Alaskan or Siberian winter, it's colder outside the front door :)
I once read advice to let frozen juice thaw completely and shake it some before drinking.