Is this a borrowing from English? Would Gwnân nhw olchi'r llestri nawr not be more accurate?
No we use this construction in Welsh too, and it is the most accurate translation of the English sentence presented.
Yes, I can see that it is an accurate literal translation of the English sentence. But the English sentence itself is deficient in that it omits some words which the hearers have to fill in for themselves. "They will do the dishes" really means "They will do (the washing of) the dishes" or more simply "They will wash the dishes". For somebody learning English who is completely unfamiliar with it, to "do" the dishes could be a bit of a puzzle.
Can this also mean that some potters will start to make dishes now, or is it only for washing dishes?