My turn to guess. Flette is no more than an old English form of "flat". Primtive languages tend to use one word to use many things so flette was a punt, or a raft, or the ground floor of a house, or a tilled field that was not on a slope.
In some German derived languages through Dutch the word "vlak" or flat can be used in many ways.
Having said that, I remember when faced with old English that spelling was erratic and words could be spelled differently by various writers. The doubled letter as to distinguish "later" from "latter" was not universal so your flette could just as easily be someone else's flete.
I have doubts about a similar word in renaissance Italian being related but I am just guessing [as I am with most of the rest of this - just feeling a bit mischievous at the moment - its my Prozac, you know].