There is no joke here. This is a biblical metaphor signifying how we turn away from obeying God. Rather, we have gone astray led by satan.
If you read the full verse, it says "All we like sheep have gone astray; we have turned every one to his own way. And the Lord has laid on Him the iniquity of us all." (Isaiah 53:6)
"All we like sheep have gone astray". This means we have all gone astray from obeying the Word of God. In other words, sinning. The analogy of sheep is used because in those times, they tended for the sheep and would usually go away from their master. This is why Jesus, in the New Testament says,
11 “I am the good shepherd; the good shepherd lays down His life for the
sheep. 12 He who is a hired hand, and not a shepherd, who is not the
owner of the sheep, sees the wolf coming, and leaves the sheep and
flees, and the wolf snatches them and scatters them. 13 He flees
because he is a hired hand and is not concerned about the sheep. 14 I
am the good shepherd, and I know My own and My own know Me, 15 even as
the Father knows Me and I know the Father; and I lay down My life for
the sheep. 16 I have other sheep, which are not of this fold; I must
bring them also, and they will hear My voice; and they will become one
flock with one shepherd. 17 For this reason the Father loves Me,
because I lay down My life so that I may take it again. 18 No one has
taken it away from Me, but I lay it down on My own initiative. I have
authority to lay it down, and I have authority to take it up again.
This commandment I received from My Father.” (John 10: 11-18)
"We have turned every one to his own way". Everyone has followed their own sinful paths instead of obeying God.
"And the Lord has laid on Him the iniquity of us all." This means that the Lord has laid all our iniquities (sins) on Him, Jesus, who died on the cross for our sins. All we have to do is repent from our sins and believe on Him, our Savior the Lord Jesus Christ.
That is what it means by we have gone astray like sheep.
When your choir director said that Handel's english was not great, that is not entirely true. There would be no way his english would not be great and still write 42 operas, 29 oratorios, and 120+ cantatas. And even if his english really was not that great, then he would have quickly learned due to his immense emersion in hymns and vocal music.
In addition, George Handel did not write the words of the Messiah. Every word of the Messiah is directly taken from the inspired Word of God. See here.
I hope that helped you address your questions. Let me know if you need any more clarification!