What is one’s mother-in-law’s mother-in-law called?
There's no single word for this in English.
Your mother-in-law is the mother of your spouse. The mother-in-law of the mother of your spouse is the mother of your spouse's father and so is your spouse's paternal grandmother.
You can call this person your 'grandmother-in-law' or your spouse's grandmother, but it is not widely understood to mean that.
You could call her your grandmother-in-law because, assuming the usual circumstances, she would be your spouse's grandmother.