I believe it was Ford who picked the song ... the only one he knew enough the words and felt confident enough to carry and dance to ...

I've just realised that Eli's last line ... "You be careful, John Book! Out among them English! ... is probably so moving because of the work done here/the divide established here.

Kiwichick, good point. And here is how the next scene plays out between Rachel and Eli, creating even more of a divide:

ELI
(in the dialect)
How can this be? How can you do such
a thing? Is this plain? Is this the
ordnung?

RACHEL
I have done nothing against the
ordnung.

ELI
(in the dialect)
Eh? Nothing? Rachel, you bring this
man to our house. With his gun of
the hand. You bring fear to this
house. Fear of English with guns
coming after. You bring blood and
whispers of more blood. Now English
music... and you are dancing to
English music! And you call this
nothing?

RACHEL
I have committed no sin.

ELI
(in English)
No sin? Maybe. Not yet. But, Rachel,
it does not look...
(tone softening... in
the dialect)
Don't you know there has been talk?
Talk about you, not him. Talk about
going to the Bishop. About having
you... shunned!

RACHEL
That is idle talk.

ELI
(in English, pleading)
Do not make light of it, Rachel.
They can do it... quick! Like that!
And then... then I can not sit at
table with you. I can not take a
thing from your hand. I... I can not
go with you to meeting!
(the old man almost
breaks down as, in
the dialect)
Rachel, good Rachel, you must not go
too far! Dear child!

Rachel is annoyed – also touched, no doubt, by the old man's
plea – but irked by his condescending tone.

RACHEL
I am not a child.

ELI
(suddenly stern again)
You are acting like one!

RACHEL
I will be the judge of that.

ELI
(fierce as a prophet)
No! They will be the judge of that!
And so will I... if you shame me!

RACHEL
(blinking a tear now,
but meeting his gaze)
You shame yourself.

And shaken – but proud and erect – she turns and walks out.