The Medieval peiod was a very, very long time before the advent of Wicca. As far as I am aware, the ceremonial tools of Wicca may include an athame -- a ceremonial dagger -- and some may choose to use a sword, but a Celtic sword is certainly not mandatory for the purpose.
The sword is used in traditional, coven-based Wicca. It is the tool of the coven as the athame is the tool of the individual Wiccan. The sword is linked to the Element of Air, just as the suit of Swords in the Tarot is linked to the Element of Air. The Element of Air is the power of the mind, the intellect. So the athame and the sword "cut" through the mists of confusion and permit the mind to see clearly.
Wicca has nothing whatsoever to do with the Celts and never has. One might have a sword with Celtic symbols on it ( which are mostly borrowed from Viking art ) but that would not make the sword itself Celtic in any way.
A couple of good sources that debunk the theory that Wicca has its roots in the religion of the ancient Celts ( And we know next to nothing about their religion ) can be found at these two links: