Not since?

Now Could you tell me, what does this sentence mean and how to rewrite this inversion sentence in its normal sentence pattern?

"Not since World War Two have so many Americans spoken a foreign language at home."

I think it means "Never have so many Americans spoken a foreign language at home since World War Two."

but when I rewrite it using inversion grammar knowledge, I find it be "Since World War Two have not so many Americans spoken a foreign language at home.", which has the opposite meaning!

To undo the inversion, you would write "So many Americans have not spoken a foreign language at home since World War II." The original (inverted) sentence, "Not since . . . ," is clearer, however.

You have understood the sentence, but this negative is very tricky - it does require care! This may make it clearer.
Since World War Two never have so many Americans spoken a foreign language at home.