Albert Barnes Commentary Psalms 37:30

"The mouth of the righteous talketh of wisdom, And his tongue speaketh justice." — Psalms 37:30 (ASV)

The mouth of the righteous speaks wisdom – That is, it is a characteristic of the righteous to speak “wise things,” not to utter folly. His conversation is serious, earnest, true, pure; and his words are faithful, kind, and just. This, as a part of human conduct, is one of the reasons why God will bless him with prosperity and length of days.

And his tongue talks of judgment – That is, of just or righteous things. .