Why does it say "v'Levav Nimharim Yavin Lada'as"?
Rashi: It is unlike it is now, that "Hashmen Lev ha'Am" (6:10).
Radak: Until now, they were Nimharim (fools) - "va'Atzas Niftalim Nimharim" (Iyov 5:13), "Imru l'Nimharei Lev" (below, 35:4). Those who made themselves fools and garblers to [not] understand Devar Hashem - "Ki b'La'agei Safah uv'Lashon Acheres Yedaber El ha'Am ha'Zeh" (28:11). After this great Teshuvah, they will return to see, hear, understand and speak clearly.
Malbim: They will not understand in order to quarrel, rather, in order to know.
Why does it say "u'Leshon Ilegim"?
Rashi: It is unlike it is now, that [they speak] "b'La'agei Safah" (28:11). Anyone who cannot speak clearly is called Alag and Nil'ag.
What is the significance of speaking clearly?
Malbim: Not only will they understand - they will also teach others.
