How were they a curse among the nations?
Radak: When hunger or evil mishaps come to their land, they say that it is due to Yisrael, who dwell among them.
How will you be a Brachah?
Radak: The nations will be blessed through you, like "ve'Hye Brachah" (Bereishis 12:2).
What does the verse teach about strengthening their hands?
Radak: Your hands will be strengthened now due to hearing the good consolations that will come upon Yisrael.
Malbim: The fifth reason why Bayis Sheni was small in their eyes is because Yisrael are disgraced and lowly among the nations. I will save you, and via this you will be Brachah. Therefore, strengthen your hands and do not slacken from building!