1)
What is the meaning of "Pi Tzadik Yehegeh Chachmah"?
1.
Rashi: First he contemplates Chachmah in his heart, to see the law that the Torah ruled for everything, and afterwards "his tongue will speak Mishpat."
2.
Radak: In addition to giving and lending, for he is generous, so he teaches his Chachmah to others, and teaches them the good path.
3.
Malbim: His mouth speaks Chachmah, and he conducts according to the statutes of Chachmah.
2)
Why does it say "u'Leshono Tedaber Mishpat"?
1.
Rashi: Refer to 37:30:1:1.
2.
Radak: He instructs with his Chachmah what he attained from Chachmos, and instructs via straight deeds matters Bein Adam l'Chavero.
