Jewish translation:
We all went astray like sheep, we have turned, each one on his way, and the Lord accepted his prayers for the iniquity of all of us.
Another one...
And we like sheep have gone astray; we have turned everyone to his own way; and the LORD has caused the iniquity of us all to fall upon him.
Israel is suffering because of other people's wickedness. The iniquity falling upon Israel is the exiles and the torturings etc.
It says lastly,
Therefore, I will allot him a portion in public, and with the strong he shall share plunder, because he poured out his soul to death, and with transgressors he was counted; and he bore the sin of many, and interceded for the transgressors.
He prays for those who transgress; good ultimately comes from this suffering. This is common. Israel is not 'taking on others' sins' as in a Christian mindset - if he somehow did this, then why would he need to intercede for them? Their sins would have gone. He is baring the burden of sinful nations by suffering under them and then ultimately this suffering turns to good, whereupon Israel intercedes for those who transgressed, as at this point in the song they have realised they have erred by persecuting him.