Ich habe das einmal von einer Bekannten gehört, im Dialog mit ihrer Tochter.

"Shall we ....?"
"Let's shall!"

Es kam so rüber, als wollte sie sich damit über etwas vornehmere Leute lustig machen, wie sie es oft tut.

It is certainly not correct English and I have never heard of this.

It's intentionally ungrammatical. I imagine it as old-fashioned, slightly upper-class BE children's talk. "Shall we?" sounds very dated nowadays, and is a bit clichéed. The answer "Let's shall" sounds like something from a children's story from the 1950s or before. If these are modern speakers, they're probably being intentionally "retro" as a little joke.

It is a goofily ungrammatic answer making fun of the use of shall, as Steve points out. You can't shall, of course, which is the point of the joke.

Das hat doch was von Karl Valentin:

"Mögen hätten wir schon wollen, aber dürfen haben wir uns nicht getraut ..."

;-)

I agree with all the comments about "Let's shall ?"

But, hey, Steve (BE), what's this about "shall we?" sounding very dated ?
If that's so, then I think I must be very dated as well.

I always thought this was just one of those things my own family said. A geflügeltes Wort or whatever. But come to think of it I haven't heard it very recently, maybe it was more my grandfather.

I actually think "Shall we?" meaning "Shall we go?" is used quite often (I actually just heard it in the Mentalist and my family uses it all the time). The other uses of shall are pretty old-fashioned though.

The Mentalist?

Yes, sorry, that's what I meant; someone says 'Shall we?' (meaning Shall we go, Shall we leave now, etc.), and someone else says 'Let's shall' (meaning, Yes, let's).

I think we've discussed elsewhere that 'shall' is less old-fashioned or formal in the questions 'Shall I?' and 'Shall we?'

I've never come across "let's shall", and was brought up on 1950s children's stories. It's a nice jokey use, though. Shall we spread the usage?

Let's shall!

As kids our response to "Shall we?" was always "We shall!