Chol Hamoed Guest,
You said, “Why should there be a Goyish Term for mechutan ‘spouse’s parents’? Such relationship dpes not exist in their culture.”
I don’t understand your question.
“Mechutan” is not your spouse’s parents. The term for THAT is your “in-laws.”
Your “mechutanim,” however, are the parents of your child’s spouse.
There indeed is no term for that in “goyish culture.”
Incidentally, I found this great website with all sorts of explanations of goyish words I wanted to share with you: