Mark, I have nothing meaningful to contribute about writing in dialects, slang, etc. as I’ve never done that to any real extent. As for reading such, I do have an opinion.
If you are reading for the sake of study and learning English literature, either formerly in a class or on your own, it is, of course, necessary to read in the language used in the original writing of the book.
Early English, say going back to 1700’s, is understandable with practice, and could be read for pleasure with some effort. For writings from earlier periods, it’s doubtful that anyone but scholars will put in the time and study necessary to comprehend what is to modern English speakers almost a foreign language.
Dialects, slang, and foreign languages are tricky. Foreign languages can certainly be skipped as a reader is not likely to be literate in multiple languages or certainly not more than two, possibly three. Tossing in foreign words or phrases that are commonly understood or easily ascertained from the story can add flavor and color and not detract from understanding what is happening or being said.
Slang that was used heavily during a period of time is certainly safe. Avid readers are likely to know such words or be able to figure out their meanings. Google is always available in a pinch. More obscure slang or very regional slang can become annoying. No one wants to stop reading every few pages to look up some slang word that a small percentage of people in the world have ever heard or used. So, in my opinion, only use slang that was or is popular and likely to be recognizable to a large percentage of readers.
Dialects are the trickiest because they add so much to a story and tell so much about the history and culture of the time. Zora Neale Hurston comes to mind. Her work would be like weak broth without dialects. I struggled when I first read Hurston, but I didn’t give up because I believed her voice was worth hearing through the written dialogs of her characters, speaking in the fashions that were natural for them. Her stories would not work without the regional and cultural dialects she used. And, once you get in the flow of dialects, they become less cumbersome.
So, that is my two cents worth, Mark! I enjoyed considering the questions you posed.