Hi Nickel and all,
I have gone back as far as Wycliffe's Bible, which predates Tyndale's by 150 years or so, and was the first complete translation into post-1066 English. Wycliffe translated from the Latin, & so looked to Jerome, I guess, for the forms of names. I do not have my Vulgate translation to hand, so cannot check the Latin.
Tyndale translated from the Greek rather than the Latin. "James" never appears as a name in an old testament context. "Jacob" appears in the new testament only in reference to the OT person. The KJV uses bits of both, tradition at that time being as important as it is now, and a good wording not to be lightly cast aside...
For more confusion, there are churches in England named St Jacob's as well as St James, but I have not been able to find out so far who they commemorate.
Jago (pron τζέιγκοου in English and γιάγκο in Cornish) is the Cornish form of the name, and is a surname.
References can be supplied when I have had a go at doing the links properly. This just in haste. Sorry for all the αγγλικά. Θανκς του γιου & αλεκς φορ δι χελπφουλ γκάιντ του ντουινγκ θιγγζ πρόπερλυ ;-)
But how shall men meditate in that, which they cannot understand? How shall they understand that which is kept close in an unknown tongue?
THE TRANSLATORS TO THE READER
Preface to the King James Version 1611