Learn English – word to describe mother in law’s brother in law

Grandmother-in-law has been accurately suggested (I linked Merriam-Webster). If you want to be specific about which grandmother-in-law, you could say "my father-in-law's mother".

You can add "-in-law" to any term of kinship to denote affinity as opposed to blood relationship.

It's in your question.

There is no special term in English for a person's child's parents-in-law.

They are your son's parents-in-law.

Now, this is not to say that English wouldn't be enriched to have such a term, but it doesn't and there we are. It also doesn't have terms to distinguish between the flavors of aunts and uncles, and a few others that do exist in other languages.