Indeed, the forensic use is established by context. To "speak evil" in this context apparently refers to testifying in a law suit against a brother so indeed James is saying the same thing as Paul: "do not sue your brother":
James 4:11 μὴ καταλαλεῖτε ἀλλήλων ἀδελφοί ὁ καταλαλῶν ἀδελφοῦ ἢ κρίνων τὸν ἀδελφὸν αὐτοῦ καταλαλεῖ νόμου καὶ κρίνει νόμον εἰ δὲ νόμον κρίνεις οὐκ εἶ ποιητὴς νόμου ἀλλὰ κριτής
I'm understanding "ὁ καταλαλῶν ἀδελφοῦ ἢ κρίνων τὸν ἀδελφὸν αὐτοῦ" to identify the κρίνων (suing) as the same person as the one doing the καταλαλῶν (accusing?) his brother. When I look at the usage of krinw it includes law suits:
[Mat 5:40 ASV] (40) And if any man would go to law with thee, and take away thy coat, let him have thy cloak also.
I posited a possible solution on B-Greek and am awaiting a critique. My solution involves a forensic usage of καταλαλέω as in an "accusation" before a Roman court. If that usage is extant or reasonable then by going before a Roman court they suggest that the Torah cannot resolve questions among saints which belittles the Torah. If my analysis is correct this becomes a parallel with Paul's reproach of the Corinthians:
[1Co 6:1-8 CSB] (1) If any of you has a dispute against another, how dare you take it to court before the unrighteous, and not before the saints? (2) Or don't you know that the saints will judge the world? And if the world is judged by you, are you unworthy to judge the trivial cases? (3) Don't you know that we will judge angels -- how much more matters of this life? (4) So if you have such matters, do you appoint as your judges those who have no standing in the church? (5) I say this to your shame! Can it be that there is not one wise person among you who is able to arbitrate between fellow believers? (6) Instead, brother goes to court against brother, and that before unbelievers! (7) As it is, to have legal disputes against one another is already a defeat for you. Why not rather be wronged? Why not rather be cheated? (8) Instead, you yourselves do wrong and cheat -- and you do this to brothers and sisters!