What does this Mizmor discuss?
Radak: It is about the days of Mashi'ach. It has only two verses, to hint that all the nations will be on two sides. Only Yisrael with their Torah are on one side, and all the nations on the other side, with their seven Mitzvos. All will praise Hashem, for all will admit to Him then - "Likro Chulam ba'Shem Hashem Le'avdo Shechem Echad" (Tzefanyah 3:9).
Why does he tell all nations to praise Hashem?
Radak: The Mizmor ends with Hallelukah, which is said to Yisrael. David saw with Ru'ach ha'Kodesh that all nations will praise Him.
Radak (2, citing Pesachim 118b): They should praise Him for what He did for them, and all the more so we should, "Ki Gavar Aleinu Chasdo" (verse 2).
Radak (2): Refer to 117:1:3:2.
Malbim: Hashem fulfilled this request, and Yisrael were saved. Also the nations that Sancheriv captured and brought in front of Yerushalayim will praise Hashem for His Chesed upon us, for via this also those captives were saved - "Yegi'a Mitzrayim u'Schar Kush u'Sva'im
Why does it say "Shabechuchu Chol ha'Umim"?
Radak: It is a repetition of "Hallelu Es Hashem Kol Goyim" in different words.
Radak (2, citing Shocher Tov): "Goyim" are the nations that subjugated Yisrael. "Umim" are those that did not subjugate Yisrael. They say, if the former praise Hashem, all the more so we should! Therefore, they tell all nations to praise Him.
