The words "the nations" and "the surrounding nations" refer to the same nations, those that surround Judah. Yahweh will judge them in the Valley of Jehoshaphat for what they have done to Jerusalem.
Jehoshaphat, whose name means "Yahweh judges," was king of Judah before Joel lived. There is no known place with this name. It would be best to translate this as the name of a valley that people named after the man Jehoshaphat. See how you translated this in [Joel 3:2](../03/02.md).