Why, then, are the Catholic Bibles translated “do penance” rather than “repent” as is the case in the King James Bible and every other so-called “Protestant” Bible?
That's simply not true.
Go to this "Bible Gateway" search for "repent" in the "Revised Standard Version Catholic Edition" of the Bible, and you will find it returns 115 instances of the word "repent", with roughly half in the Old Testament, and half in the New Testament.
(The "Revised Standard Version Catholic Edition" of the Bible is likely the most widely used English version of the Bible by Catholics today.)
In the Greek, the word is the word for *repent* not *do penance*.
Putting in *do penance* is just another not so subtle mistranslation of the Greek to make Scripture say what the Catholic church wants it to say.