Fountain of Knowledge

Here's my question/what I need an opinion on:

In stories where the characters are speaking a foreign language or with an accent, it's pretty common to just translate that into the same language as the book, for the sake of the reader. For instance, French characters - it's assumed they're speaking French, but their dialogue is in English in a book written in English.

So can I assume dialect works the same way? As a specific example, what I'm writing takes place in ancient Syria (4,000 BC or so). Can I assume that if I show my characters' dialogue in English, that I have a certain amount of freedom in their dialect as well? Or do I destroy something by not using a more formal tone for something so ancient? (I'm not really considering writing 20k words of thee's and thou's if anyone is curious)