I don't know anything about snowboarding, in Iceland or anywhere else, so can't help you on that account. Also, I've never been to Iceland, in summer or winter. So why the heck am I replying? Well, having visited a couple of different northern European countries in the dead of winter, I can at least suggest that you probably won't be totally alone in the hostels - often locals travel at that time of year, which gives you a great chance to meet people who speak the language or might invite you to their homes in other areas. Also, any "foreigners" who happen to be around will likely be long-term travelers, and you'll be amazed at the stories they have to tell, and the knowledge they've gained.
Meeting people at the snowboarding venues should also be fairly easy - you automatically have a shared interest, and during the off season people tend to be a bit more open to strangers than during the frenetic high season. Keeping in mind that in general, northern Europeans tend to come across as a bit more aloof than southern Europeans or Americans, for example, in our experience once you get through the initial barriers, you can make some of the greatest friends.