This will was written in 1690 and was from a Hugenot ancestor of mine in South Carolina. It wasn't a matter of "thou understandeth" exactly. It was more a matter that words used had long ago fallen out of common usage. Like the word "sere" has fallen out of common usage in English. There were words for farm implements and stuff that the translator couldn;t recognize.
I had the same problem deciphering a tombstone inscription of another ancestor. This one was an 1863 tombstone in Tennessee. The woman died in childbirth while her husband, an army physician, was away serving in the Civil War. He was an Abernathy, from Scotland. WHen she died they let him come home to bury her and the child, and he decided that he wanted to make this really romantic gesture and he wrote her tombstone inscription in SPANISH of all things.
I speak a small amount of SPanish, I used to be fairly fluent but that was long ago...but I knew enough to try to translate this using Babelfish....but it kept making NO SENSE.
Finally I gave it to a fluent Spanish speaker and the problem was it had been translated from English to Spanish as a "literal translation", he said probably the person who wrote it used some sort of Spanish dictionary and looked up each individual word and wrote it all down "literally" so when it gets translated back it has very odd usage and connotation.
Can you imagine how weird that would be, to be walking through a Civil War era cemetery in rural Tennessee and come upon a tombstone written in SPANISH?