Baha'u'llah did not speak in early 17th Century English, He spoke in Persian and Arabic.
His Writings were translated in that way.
Some time ago,
@TransmutingSoul explained why the Writings were translated into King James English.
Tony said: The reason is that Shoghi Effendi went to England to study English so He could better translate the Writings of Baha'u'llah from Persian and Arabic into English.
From his studies he determined that King James English was the best form to portray Persian and Arabic to English speakers.
Apparently Persian and Arabic have a form of poetic prose that is hard to portray to English speakers. King James English must in a small way convey some of that poetic prose experienced by Persian and Arabic speakers.
Shoghi Effendi offered that the future may see different translations.