With our exchange students, they are all college aged and over the age of 18 so liability in what they choose to do isn't a concern at all. At that point it's their problem. And since they work on our farm they come with their own health insurance and liability insurance. Also with our farm since it's a business we have our own liability policy as well.
So for the younger ones, I don't really know TBH. I will say I work in a school and it seems the exchange student programs all have their own set of rules and requirements, which usually place very little liability on the hosts. It's more of a shape up or ship out sort of thing.
As for the English we're on E. student number 4 and about 50/50 on the language skills. 2 have spoken better English than most Americans, 1 was just avg. and 1 downright horrible compared to the rest. However, we always found a way to communicate, whether be writing it down, hand signals, google translate for the jist etc. I wouldn't English ability deter you at all...you catch on to what they will and will not understand pretty instantly. Esp. for just a week...I wouldn't consider it a big deal at all. I've also learned that the kids that choose these type of programs are really intelligent, outgoing and truly want to participate so they will make every effort to communicate.
Talk to your program coordinator. They're usually a wealth of great information. Sorry, I couldn't help you more but we work with such a specific program within a university system and an exact skill set and age range, so we face a whole another set of challenges that "regular" exchange student families don't and vice a versa.