Folly may refer either to something stupid that results from the disregard of God’s will (1 Sam 25.25) or to some grave sin (Gen 34.7; Josh 7.15 [Revised Standard Version “shameful thing”]). The second interpretation seems more likely here, and most versions have something such as “vile things” or “terrible sins” (so Good News Translation). These sins were adultery and telling lies in the LORD’s name. This is the only occurrence of the Hebrew term in Jeremiah.
Committed adultery with their neighbors’ wives is translated “committed adultery with other men’s wives” by Revised English Bible; Good News Translation has simply “committed adultery.”
They have spoken in my name lying words which I did not command them is more literally “they spoke a word in my name a lying one which I did not command them.” Once again it is quite obvious that the Hebrew text cannot be rendered literally without making nonsense. Revised English Bible is rather lengthy: “and without my authority prophesied in my name, and what they prophesied was false.” New Jerusalem Bible renders “speaking lying words in my name without orders from me.” Die Bibel im heutigen Deutsch uses an idiom: “by appealing to me, they spoke lies which I did not place in their mouth.” Another way to express it is “they have given the people messages that they said came from me. But what they said was a lie and I never told them to say these things.” For “speaking in the LORD’s name,” see 20.9.
In the construction I am the one who knows the pronoun I is emphatic, so Die Bibel im heutigen Deutsch has “I, the Lord, know what they have done.” In many languages the verb knows will require an object, as in “knows what they did.” It would seem that the two statements I am the one who knows and I am witness mean essentially the same thing, each intended to reinforce the other. Good News Translation (referring to the LORD in the third person) translates “he knows what they have done, and he is a witness against them.” Die Bibel im heutigen Deutsch is essentially the same as Good News Translation, except that the first person is retained. As the Good News Translation footnote indicates, the text may also be interpreted to mean “… done; he saw them do it.” However I am witness can be understood as a statement a person might make on a legal document, so translators can say something like “I will state that this is true” or “I can tell that I know this [to be true].”
Says the LORD: Good News Translation, which makes this into an independent sentence, is more emphatic with “The LORD has spoken.” See 1.8.
Quoted with permission from Newman, Barclay M. and Stine, Philip C. A Handbook on Jeremiah. (UBS Helps for Translators). New York: UBS, 2003. For this and other handbooks for translators see here .