You know how in fantasy worlds, its all english? Kinda breaks the immersion a bit. I wanna find something where they make it as realistic as possible, and make everything in a fictional language, basically using subtitles as the main way to understand the plot.
It’s not fully the type of answer you want, but there is an Italian book called “the revolution of the moon” that is 90% written in dialect. The first pages are mostly Italian with some words in Sicilian dialect, then the dialect part gets more and more prevalent until it’s only dialect.
It’s not exactly what you mean in the sense that the Sicilian dialect really exists and that the book clearly exploits the similarities between the dialect and Italian for the reader to understand.
To stray even further from OP’s question (because books), I loved the dialect in Riddley Walker and the slang in A Clockwork Orange.