Do you think they’d be able to understand Shakespeare? Or a spell book from the same time as Shakespeare?
The bigger question is, do they need to understand Shakespeare?
Shakespeare is important in UK Muggle culture, not so much elsewhere, including the wizarding world which very much has their own cultural icons.
They probably aren't getting their spells from books that old either. The reason we don't transpose Shakespeare to modern language is the loss of artistic intent in the process. Something that wouldn't apply to purely factual books like a spell book.
Besides, they are expected to learn Latin; that's where their spells come from.