... And who says Merlin's journals are in Welsh? If I had to bet, I'd say they were more likely in Greek or Latin. Whatever was the lingua franca of the time.
In the Mediterranean, sure!
But not in the British Isles. Almost no Greek, and only a small amount of Latin, which was dying out post-conquest as the Romans had largely withdrawn by 410AD.
Arthur was in the tumultuous post-Roman era. Latin was limited to being a liturgical/church language (which emphatically was
not Merlin's background or allegiance! Folklorically, Merlin came from the people
FIGHTING the Romans (and the Christian church!))
If we want to get pedantic: Old Welsh is probably
too recent to be Merlin's mother tongue!
The Celtic arrival in Britain is recent enough that their language hasn't yet split up much... the Picts still control Scotland, it isn't "Scottish" (Celtic) at all, and the "Welsh" are still culturally and linguistically contiguous with most of southern Britain.
Merlin spoke...
... wait for it ...
"British." That's one of the (several) names given to the language spoken by the tribes of Celts who were all "Britons." Aka "Old Brythonic" and "Common Brittonic" and several other labels.
The Welsh language fragmented from Brythonic into what became "Archaic" or "Primitive" Welsh, which then became "Old Welsh," which I think becomes the Welsh "mother tongue" about 800AD, which is AFAIK well after the Arthur/Merlin era.