"One could say that..." means something similar to "in other words", or "another possible way of saying this is ..."

You can think of it as "There's a possibility that someone will say (that) it's spoken differently in every region". Here, 'Could' implies a possibility. Hope it helps!

I usually like what Gillian writes but would beg to differ this time. "One" is not archaic. One uses it all the time.

You might say that, or some people may say that, it is possible to say that. Just to add using one is now quite archaic and old fashioned in most English speaking countries. Instead we say, You could say, English is often spoken with a different accent in every region in Britain.