[Olog-hai]

I forget which region of England prefers saying “Don’t lets” instead of “Let’s not”.

[alexander_busek]

I forget which region of England prefers saying “Don’t let's” instead of “Let’s not”.

It is not a so-called "regionalism."

Rather, it is a "cute" or "marked" expression. Might have originally been derived from a certain dialect or a certain sociolect, like "Is you is or is you ain't my..."

Regards,