Nahum changes how he uses the image of locusts halfway through this verse. Now he compares the people of Nineveh to locusts. Multiply yourselves like locusts.
In other words, multiply your army in the hope that it will withstand the Babylonians. But of course, it will not.
15 Là, le feu te dévorera, L'épée t'exterminera, Te dévorera comme des sauterelles. Entasse-toi comme les sauterelles! Entasse-toi comme les sauterelles!