Jamieson, Fausset and Brown on Joel 2:17

17. between the porch and ... altar--the porch of Solomon's temple on the east (1Ki 6:3); the altar of burnt offerings in the court of the priests, before the porch (2Ch 8:12; compare Eze 8:16; Mt 23:35). The suppliants thus were to stand with their backs to the altar on which they had nothing to offer, their faces towards the place of the Shekinah presence.

heathen should rule over them--This shows that not locusts, but human foes, are intended. The Margin translation, "use a byword against them," is not supported by the Hebrew.

wherefore should they say ... Where is their God?--that is, do not for thine own honor's sake, let the heathen sneer at the God of Israel, as unable to save His people (Psa 79:10; 115:2).