Hm, to be honest, Old English's linguistic status in Merlin in particular is pretty much irrelevant - according to BBC, Merlin is set in a fantasy alternative universe from reality, so a lot of the historical facts can be completely ignored.
On one hand, there is all this stuff in the show that sets it in the Middle Ages, but the show itself uses the traditional story lines and legends of Merlin - set in the 6th century - as opposed to the more popular ones of the later centuries. i.e. it's actually only the later versions of the legends that have Merlin as an old man - in the earlier versions, he was a young boy genius. (I'm sure our Arthur would despair to hear that!) Morgana was Arthur's adopted-sister in the original legends (as opposed to half sister of later legends) and was, in fact, Arthur's ally - if a somewhat distant one - rather than his enemy...and ultimately, Emrys, which is the original/Welsh version of the name. These versions of the legends took place in the century or two after the fall of the Roman Empire in Britain.
Re: praise! ... and pedantry
On one hand, there is all this stuff in the show that sets it in the Middle Ages, but the show itself uses the traditional story lines and legends of Merlin - set in the 6th century - as opposed to the more popular ones of the later centuries. i.e. it's actually only the later versions of the legends that have Merlin as an old man - in the earlier versions, he was a young boy genius. (I'm sure our Arthur would despair to hear that!) Morgana was Arthur's adopted-sister in the original legends (as opposed to half sister of later legends) and was, in fact, Arthur's ally - if a somewhat distant one - rather than his enemy...and ultimately, Emrys, which is the original/Welsh version of the name. These versions of the legends took place in the century or two after the fall of the Roman Empire in Britain.
:D