It may actually depend on the family you will stay with. Not all Japanese speak the same way. In some families, the parents expect their kids to speak to them very formally. But in many others, you can talk to your parents as if they were your friends. So start with the formal kind of speech that I'm sure you're learning right now. If the mom and dad don't ask you to speak more informally, you won't. If they do, you will.

This happens to not only foreign guests. It happens to us, too. When I meet someone as old as or older than myself for the first time, I tend to speak fairly formally. They often ask me to speak less formally as we get to know one another.

So, unless you stay with an exceptionally 'formal' family, it wll be a great opportunity for you to learn informal/colloquial Japanese.

In class we are doing formal lessons but i aquired a set of informal only audio lessons which was well worth it because to me, infromal still sounds like a compleetly different language sometimes.. I'm learning though. Currently doing masculin vs feminine stuff. Finding it hard to get my head around what to use when and where. Things that I wouldnt have been bothered about saying 3 weeks ago now sound feminine? if that makes sense.. and im really worried about what I should be saying and it just puts me off talking altogether.. meh, im only on lesson 8 out of 150 in this pack so at the end i might sound like one of your locals then i will be "scraping you off the ceiling" as i believe you put it :p