I see where you're coming from. It sounds as if, possibly, your husband is himself from another culture -- correct? If so, is it a culture where it's considered perfectly normal and no big deal for people to stay with relatives for extended periods? Culturally, he might feel that not only is it no big deal, it IS a big deal to say no or suggest a hotel, etc. I'm not saying that this is an excuse; I agree with you that this sounds like potentially a lot to take on. But it may provide an explanation for why he seems to be so "checked out" and just not hearing you when you say you have these concerns. It's time for him to hear just how checked out he is and to know that if he doesn't start to take a role, he's affecting his marriage negatively.
I would sit down with him at a time when there are no kids around, your mom isn't around, no TV in the background, he has nowhere he needs to be in half an hour, you aren't all on the way out the door to somewhere -- and tell him in that setting, face to face, that you need to lay out some issues and need him to take your concerns seriously and take certain ACTIONS you are going to discuss, so that the visit can happen -- but so that you and he are not having marriage trouble once it's over! The fact he also has a buddy coming to stay next week indicates he is willing to open the house readily, and if this irritates you, it's time to discuss it with him before you have a stream of folks staying.
Be ready to offer something positive rather than just say how this bugs you (though again, I agree that it does have aspects that would bug me too -- the length of the stay, the expectation that you are the entertainer-in-chief and chauffeur and food provider, and the expectation that you will take the guests to the doctor!). Just saying you're not happy with this isn't working so I would get specific about actions he has to take.
Positive here might be along these general lines: I know this visit is set. I want to see the relatives and get to know them. But I need some ground rules, and some things dealt with before your SIL gets here. First, I'm concerned that she will get sick while here. So you need to phone her now, in her home country, and ensure she brings whatever medications she needs, copies of records translated into English before she arrives, whatever. She and you have to talk to whoever you need to, to ensure that any meds she brings are OK to bring into the U.S. You need to talk to her in detail so she understands that in the U.S. it can be hard or impossible just to walk into a doctor's office; she does not have medical insurance here. If she is sick she will end up in a hospital emergency room, not a doctor's office. She needs to know this isn't a time for her to "shop" for different answers from different doctors. Also, she needs to know that I plan to do...(here, you fill in the blank: You have your own appointment, or social plan, or prearranged meeting with a friend, etc. during a few of those days. In other words -- you have prearranged "getaway time" when you do not have to be with the guests)."
If your husband can't get off work while they're here -- and I think he should try, or they should have waited until he had vacation time and come then -- tell him he has to organize a couple of outings, even if you're the one who chaperones those outings. HE has to get online at night before they come and arrange some things, make some reservations, and hand you the details. Make him take some level of responsibility for entertaining the guests.
You may get lucky; they may actually expect less entertaining than you fear, especially if the SIL wants mostly to relax.
The language issue is the thorniest one for you. If she gets sick and the language is not one where you might find someone readiily who speaks it (like Spanish), she should have a card or document she carries all the time that explains her condition, and she should get it translated into English in writing and carry that. You will not be able to translate for her if she gets ill suddenly.
I hope I don't sound down on this woman - I actually feel awful for her because hives can be very, very painful and debilitating to the point of depression, and she sounds like she is holding out hope of feeliing better while with you. I know you'll make her feel welcome and not take the anger you feel toward your husband out on her. But I do feel that your husband is on a trend of expecting you to host his friends and family, and if that's the case, he has to do it when he can take on as much of the work as you do.