Riddley Walker by Russell Hoban.

Quoting from wikipedia:

The book is set “roughly two thousand years after a nuclear war has devastated civilization … [and] … is written in a stylistic, imaginary dialect based on and inspired by the dialect of Kent”

“The struggle with Riddley's language is what makes reading the book so absorbing, so completely possessing."

It takes a while for your brain to adjust, but once it does, you don’t even notice.