Translating modern English into medieval English is not easy, and requires a good deal of knowledge of the medieval language. Also, it depends on what you mean by medieval English.
There are different kinds of medieval English. One is Old English, which is also called Anglo Saxon. To the modern eye, this looks nothing whatever like the English we are used to; it seems more like a Viking language or some such thing.
Later, Old English developed into Middle English, which is much closer to what we speak today. Here is a sample:
Whan that Aprill, with his shoures soote
The droghte of March hath perced to the roote
And bathed every veyne in swich licour,
Of which vertu engendred is the flour...
In modern English, this is as follows:
When April, with sweet showers,
Has pierced the drought of March to the root,
And bathed every vein in such liquid,
That its power has brought into being the flower...
Clearly, this is not something that can be done without understanding it.