I'm writing an urban fantasy set around 1927 in the US and I've run into some of these issues.
In many ways I think there are two factors going into a reader's mind:
Modern Concepts and Historical Events
In my story I've had to reword sections or even remove scenes if I accidentally referred to a concept or invention that was created later than 1927. For example, the protagonist was a wizard in Atlantis, 6,000 years ago who has been reborn into this modern age. In Atlantis he created sorcery which is very similar to a programming language. And I've had to reword around that several times because the modern computer language hadn't been invented yet! In fact, "computer" was a job title at the time.
Phones weren't as common as they are today. People wore watches if they needed to know the time, and they had to wind them for them to keep working.
I have to keep remembering that picking up some beer was a much more difficult thing at them time because alcohol was illegal.
So referring to anything like this will jar a reader who knows history.
Reader's Expectations of the Language
This is difficult to define, because it differs depending on the reader and the story. Set in fifth century Persia, readers might expect some language to sound like the movies and stories set in roughly that area and time ("Kismet" and "The 2001 Nights"). However, most of the language will be in modern English to make the story accessible to modern readers.
You should do a lot of research as to how much formality there is in common situations. How would a son refer to his father? How would a husband refer to his wife in public and private. In my story, when the protagonist was married in Atlantis, in public they referred to each other as "husband" and "wife", or "my husband" if referring to him to somebody else, never by name. In private, they use their first names. I took this practice from a field-linguistics course I took in college where we had to learn about a culture and language from a native speaker from India. When I was growing up, it was still common in some places for a married couple to refer to each other as "mother" and "father," but it seems less common today.
I also took slang from old movies set during that time period, as well as some internet sources. I use this occasionally when it seems appropriate.
I'm lucky in that I can invent my Atlanean culture, but I did have to do this. And readers do expect a "fantasy" feel to Atlantis because it was filled with magic (at the time, in the modern era, the fey have become more careful about using magic around humans).
I set a story in the late 1970's and I had to reword a few sections that referred to microwaves as they weren't common at that time. They were available, but they were uncommon. Not to mention one section where one character called another on her cell phone. Ooops. Rewrite!
The biggest thing I would look out for is modern concepts used in language. "Spoilers" seem to be one of them. In fifth century Persia I would expect that most stories were spoken rather than written and that books would be uncommon and expensive.