Why is a Tzadik's tongue compared to choice Kesef?
Rashi: He knows how to rebuke.
Orchos Tzadikim (introduction, DH v'Yesh Adam): Even if Reuven recognizes Shimon, he will not find him if he does not search properly. If one comes to collect from one who owes him, if he does not know the accuracy of the scale or weights, or is not expert to recognize [pure] silver, or does not look carefully, he can suffer a loss. If he is careful about all of these, he is paid properly - "Kesef Nivchar Leshon Tzadik."
Malbim: The tongue is attributed to Binah, the mouth to Chachmah, and the lips to Da'as. The tongue hints to internal speech, matters that he investigated in the ways of Binah. Leshon Tzadik is what he contemplated about Tzedek. It is like choice silver without any dross. All Binah of Nevonim has dross; he considers everything that contradicts, until he finds what is proper; the excess is dross. However, a Tzadik, who is used to doing Tzedek, and contemplates it with practical Sechel to do in practice. Every particular matter that arises, he understands immediately from experience how to do it without any doubt. 1
Malbim: Safah refers to external speech; this is only in matters of Da'as, which he knows clearly, like what he sensed. He can speak about them without thought. The mouth is intermediary between the tongue and lips, it refers to matters of Chachmah that he received from his Rebbeyim. He does not understand them like matters of Tevunah, and they are not fluent on his lips like matters of Da'as.
What is the meaning of "Lev Resha'im Kim'at"?
Rashi: He does not heed a Tzadik's rebuke.
Rashi citing Midrash Tanchuma: This refers to Edo ha'Navi. He called on the Mizbe'ach in Beis Kel. Yaravam, even though his hand dried [while offering to idolatry], he did not accept rebuke - "Chal Na Es Pnei Hashem Elokecha" (Melachim I, 13:6), and not my god. "Va'Tashav Yad ha'Melech Elav va'Tehi keva'Rishonah" (ibid.), and he continued to serve idolatry.
R. Yonah: He does not rule over his Ru'ach to settle his thoughts to clarify matters. He rushes to speak before speaking to his heart. Kim'at is a brief moment, like "Kim'at Oyveihem Achni'a" (Tehilim 81:15), "Kim'at ki'Sdom Hayinu 1 " (Yeshayah 1:9).
Malbim: Resha'im are far from Tzedek. Their heart - the power that rules in them, is not always on one track. He constantly wavers to another Midah; his heart is like Kim'at - he has no heart to rule at all. All the more so he does not do absolute good, for he has no special way at all.
Why did he bring this verse, in which one could explain that Kim'at means almost, and not "Chavi Chim'at Rega" (Yeshayah 26:20)? Perhaps he wanted to teach, Agav, that also "Kim'at ki'Sdom" refers to a moment. (PF)
