sworn by myself - equivalent to, “As I live,” as Rom 14:11 quotes it. So Num 14:21. God could swear by no greater, therefore He swears by Himself (Heb 6:13, Heb 6:16).
word ... in righteousness - rather, “the truth (see on Isa 45:19) is gone forth from My mouth, the word (of promise), and it shall not return (that is, which shall not be revoked)” [Lowth]. But the accents favor English Version.
tongue ... swear - namely, an oath of allegiance to God as their true King (see on Isa 19:18; see on Isa 65:16). Yet to be fulfilled (Zec 14:9).