י֧וֹם חֹ֣שֶׁךְ וַאֲפֵלָ֗ה י֤וֹם עָנָן֙ וַעֲרָפֶ֔ל כְּשַׁ֖חַר פָּרֻ֣שׂ עַל־הֶֽהָרִ֑ים עַ֚ם רַ֣ב וְעָצ֔וּם כָּמֹ֗הוּ לֹ֤א נִֽהְיָה֙ מִן־הָ֣עוֹלָ֔ם וְאַֽחֲרָיו֙ לֹ֣א יוֹסֵ֔ף עַד־שְׁנֵ֖י דּ֥וֹר וָדֽוֹר׃
那是黑暗陰沉的日子，是密雲濃霧的日子；有一群眾多而強盛的民，如晨光鋪展在群山之上。自古以來未曾有過像他們這樣的，此後直到世世代代的年歲，也不會再有。
Rashi
like the dawn, spread over the mountains—The increasing locusts and the shearing locusts are spread over the mountains as the dawn is spread over the entire world.
Abarbanel
1. and it mentions the day of the destructions that it will be a day of darkness and dense clouds, smoke and gloom. These descriptions all apply to the destruction of the first Temple and the troubles that came along with. And on this Ivoy (3:5) says about the day of his birth "may darkness and gloom claim it and a cloud lie on it". And maybe it is a hint to the day that is decreed for our troubles that is the 9th of Av. Because that is the day of Gd that he put aside to take vengence on His nation and it was darkness and gloom for the generation who was in the desert because on that day the decrees of the spies took place. And it was a day of clouds and smoke when the Temple was destroyed. And it is say it is spreaking on the mountains meaning that it is like there was no light in the world except the darkness and that was spreading on the mountains. The word in the pasuk "shachar" is from the root of the word "black" and therefore the daybreak is called "shachar" because the light has not yet started to spread on the world. And the meaning is the darkness hints to a great multitude of a nation that Nevuchad nezer brought to Jerusalem that was great in number and mighty in stregnth and there was never before like it in the world and never again afterwards for gerenations. This means that from the time the world was created, no nation had ever gone to war with such a great and mighty army like him and afterwards this will never be for generations. even the army led by Titus to destoruy the second Temple was much larger than Nevud Nezers and therefore it says "for generations" since Titus's army was many generations later after 500 years.
Rashi
the like of which has never been—that all these species should come one after the other, but the species of arbeh alone did not equal that of Egypt, concerning which it is stated: (Exodus 10:14) “And after it there shall not be so.”