My idea for a novel about baseball is simple. This story is about being a baseball fan around 1920. In the name of conveying the way my great grandfather would have seen a baseball game, to what extent should I include the slang of the period? The goal of the dialogue is to do more than recreate his experiences. I also hope to convey his excitement over events on the field, and to convey what it was like to discuss and argue over baseball as fans would have done at the time. Since this is set during the 'deadball era', before Babe Ruth and his home runs, baseball slang of the era would have to be included, I would think. My concern is the use of era appropriate slang from American culture at large. Is that necessary at all, or to what extent?
This is a common problem for period writing. You are writing about 1920, so your only problem is a slang. Imagine if the English itself was different in your period.
You will need to strike a balance between linguistic accuracy and readability. Fiction writers usually prefer readability over accuracy, because most of their readers prefer that too. So, you cherry pick a few slang terms and use them throughout your book, giving it certain "flavor". You do need to explain those terms, though, and using too many may distract some readers.