Who will judge?
Radak: The judge, i.e. Melech ha'Mashi'ach. [The subject is omitted,] like "va'Yaged l'Yakov" and "va'Yomer l'Yosef" Bereishis 48:2, 1) - the one who told him, and similar verses.
Why does it say "Bein Goyim"?
Radak: If there will be war or claims between nations, they will come for judgment to Melech ha'Mashi'ach. He will be master over all the nations, and will rebuke them. The one who did improperly, he will tell him to fix his misdeed against his opponent. There will not be war between nations, for he will make peace between them!
Malbim: This is like they requested [someone to instruct them - verse 3]. There are two reasons for war. (a) Between different nations, since there is no creed between them, or each has its own creed, the sword decides between them. (b) Guarding authority and order within a country, lest people rebel. These reasons will cease, "for he will judge between nations and rebuke." His rebuke will suffice to stop nations; they will not transgress the law.
What are "Itim"?
Rashi: They are plowshares.
Radak: They are shovels used to dig.
What are "Mazmeros"?
Rashi: They are scythes used to prune vineyards. Radak - this is like "v'Charmecha Lo Sizmor" (Vayikra 25:4).
