The Jahwist, as in the hypothesised earliest source document of the Torah? I’m dismayed to find from Wikipedia that the documentary hypothesis is now falling apart, and increasingly scholars think there was no one unitary Jahwist document. Doesn’t matter to me if the Jahwist was a bunch of bits; that bunch of bits is still more interested in Judah than Israel, and still features Yahweh as a petty anthropomorphic figure.
The classic documentary hypothesis dates the Yahwist to the 10th century BC, although that too is now out of favour. That would certainly be the earliest date for a Yahwist corpus.
So the question becomes, what was the language of 10th century BC Judah?
Still Hebrew, although you can legitimately argue about how distinct palaeo-Hebrew and Phoenecian were at that early time. It’s the time of the Gezer calendar, discovered 20 miles west of Jerusalem.