I am sad that you are finding your visit difficult. As you are only there a short time there are a few things you can consider to make the rest of your time with your family a little easier.
1) As others have said your DIL may be trying to give you time with your son.
2) She may be running from one end of an exhausted day to the next and doing the best she can. Working at a boarding school she may have "house duties" that keep her at work after the school day. So her going to bed could have lots of reasons, exhaustion being one of them.
3) Google "Culture Shock" your post reads as if you are suffering at least some of the effects. You do sounds as if you are feeling anxious and somewhat off your normal balance - you are allowed to feel these feelings. Be a little kinder to yourself and in turn this will help you relax. It really is worth a quick read around "Culture Shock".
4) As sad as it is that you and your DGC are struggling with the language you are there with them. Body language speaks volumes and children are the best at reading it as I'm sure you know so best face forward - to misquote the saying.
5) Examine the expectations you had about your time there.
Look hard at what ideas you put in your mind about How your visit Should be.
Look at What you Expected their family structure to be, structure of time, how they parent, what time would they normally have as a family on a working day if you weren't there etc.
Have a think about those things and then look again and see if in all their daily tasks they have tried to make some space for you.
I'm sure they have and so are you, you have given them some credit so you are finding your balance again.
However, not only is the country culture different but their family patterns sound very different to the patterns/structures you had when you were parenting your son.
It is the most natural thing in the world to go into a situation thinking it will be very similar to one we've experienced in the past only to find this situation is very, very different and that will throw us off balance.
6) However sad and not yourself you are feeling right now it is clear you are doing your best to make it work. Give yourself credit for your efforts at this tough time, be kind to yourself, that kindness gets passed round.
And "This Too Shall Pass". As you know only too well this time will pass very fast so you clearly want to make the most of it for you, your son and the DGC as well as making it as easy as possible for your DIL.
Good Luck and may you find joy in the rest of your visit.