I am so sorry. I have been in this situation, without the different country thing. I wanted my MIL to know she was welcome in her GC lives, but I didn't want to have to tell her why we were divorcing. I had a difficult MIL and I knew that if I told her, she was not going to believe me. But, I also didn't want her son telling her lies and thinking that I was taking the kids from her.
So, I choose both. I called her and simply said, "MIL, your son and I are getting a divorce. I wanted you to know before you heard it around town and I also wanted to thank you for everything you have done for me. I want you know, you can call me any time in regards to the boys and see them, but I will not discuss the ending of the marriage with you. If your Son wants to discuss it with you, then that is up to him."
Now, I had a difficult MIL and she immediately starting saying, what happened? I can't believe this, what is going on? I politely told her that she would have to talk to her son, and got off the phone. The next two weeks, oh lord. She gave me ugly looks all over town. People were telling me that she told them, "There is no telling what she did this time! I'm glad he finally got smart!" Etc.....
One month later, she called me remorsefully and said, "I am so sorry. I had no idea." I was able to reinterate that I was thankful for everything and to call me to arrange to see the boys. I found out that she kept asking her son what had happened and he wouldn't tell her either, so she assumed it was me. When he was seen around town with his 20 year old "new" girlfriend, people finally started talking and she figured it out when he brought her over to meet them. When she found out, it kind of kicked her in the gut after talking about me for those weeks. Now, in typical difficult MIL style, it turned to "Well, I don't agree with how he did it, but I guess if she had treated him better...." I can laugh now, because that was always how she had been. But, she does still call me every once in a while (my boys are adults now) and checks in. I think deep down, she knows I didn't do anything, but will not admit her baby boy did anything wrong.
I think you should call her, and not go into details, but let her know you are leaving and that she is still welcome in the GC's life. If she asks why, I would leave it with, "Your Son and I are having issues and you will need to discuss that with him." She may not understand at first, but eventually, it will surface.