Jamieson, Fausset and Brown on Isaiah 66:18

18. know--not in the Hebrew. Rather, understand the words by aposiopesis; it is usual in threats to leave the persons threatened to supply the hiatus from their own fears, owing to conscious guilt: "For I ... their works and thoughts," &c.; namely, will punish [Maurer].

it shall come--the time is come that I will, &c. [Maurer].

gather ... nations--against Jerusalem, where the ungodly Jews shall perish; and then the Lord at last shall fight for Jerusalem against those nations: and the survivors (Is 66:19) shall "see God's glory" (Zec 12:8, 9; 14:1-3, 9).

tongues--which have been many owing to sin, being confounded at Babel, but which shall again be one in Christ (Da 7:14; Zep 3:9; Re 7:9, 10).