What is the meaning of "Sifsei Tzadik Yede'un Ratzon"?
Rashi: They know how to appease their Creator and people, and to make Shalom among them.
R. Yonah: After saying "Pi Tzadik Yanuv Chachmah" and cursing "Leshon Tahpuchos", our verse teaches that even though a Tzadik speaks only Emes, he does deviate for the sake of Shalom. His lips are accustomed to appease his colleagues - "Ma'aneh Rach Yashiv Chemah" (15:1). One may deviate for the sake of Shalom (Yevamos 65b). If not for this, one would not be able to bring Shalom between people, which verses and Chachamim praise.
Malbim: I explained above (21) that a Tzadik reached Da'as in details of the ways of Chachmah. He has a clear knowledge; lips represent external speech (he need not think, for it is already clear to him). Ratzon refers to being pleasing to Hashem, and appeasing anger. A Tzadik knows what to say that is the desire of Hashem and of man - what is glory for Oseha 1 and glory for him from people (Avos 2:1). "Yede'un" implies clear knowledge.
Malbim explains Oseha to refer to Hashem; many say that it refers to the one who does the act. (PF)
Why does it say "u'Fi Resha'im Tahpuchos"?
R. Yonah: Their mouth is accustomed to lie and conclude the opposite of Emes in every argument, even if it does not affect them and they have no benefit. They support the false claim and support it. What brings them to this? Since they always did injustice, oppression and theft, and they always needed to make false claims in their disputes with their colleagues, to say 'I did not do wrong', their lips became used to calling bad 'good' and vice-versa, and to decide like Sheker.
R. Yonah (31): Resha'im conduct opposite to the Tzadik's Midah, whose mouth bear fruits of Chachmah.
Malbim: Their mouth speaks of conduct the opposite of Chachmah, as if they received laws opposite to the ways of Chachmah.
