This saying is not easy to interpret and translators differ in their expression of it. It is necessary to examine both lines to clarify the problem.
“Deceit is in the heart of those who devise evil”: “Deceit” translates the same word used in line 2 of verse 17; however, here the reference is general and not connected to a court witness. In this context “Deceit” appears to be the condition of the heart (the purpose or intention) of those who “devise evil” (plot or scheme to harm others).
“Those who plan good have joy”: This is not what we might expect to find in this line. After line 1 we expect something corresponding or contrasting to “Deceit in the heart”, such as “honesty in the heart,” that would express the intentions of good people. “Plan” translates a word meaning to advise or give counsel. “Good” translates the Hebrew word shalom, so “plan good” means to advise people to live in peace. “Have joy” means to be happy.
Some interpreters modify the Hebrew in order to get a clear contrast in line 2. However, others get an adequate contrast by interpreting “Deceit” in line 1 to refer to “self-deceit” and so Scott, for example, translates “Those who plot trouble are deceiving themselves, but men who counsel peace will be happy.” Note that Good News Translation follows the same interpretation, as do New English Bible/Revised English Bible: “Those who plot evil delude themselves, but there is joy for those who seek the common good.”
Going back to the previous interpretation, it is also possible to contrast “Deceit” in line 1 with “joy” in line 2 by translating, as New Jerusalem Bible does: “Deceit is in the heart of the schemer, joy with those who give counsels of peace.”
We may also say idiomatically in some languages, “People who plot to harm others have double hearts, but those who work for everyone’s good have glad livers.”
Quoted with permission from Reyburn, William D. and Fry, Euan McG. A Handbook on Proverbs. (UBS Helps for Translators). New York: UBS, 2000. For this and other handbooks for translators see here .