"Dinner is being ready" is not natural English. "Dinner is about to be ready" is more feasible, but doesn't feel the most natural to me either. I would say "Dinner is almost/nearly ready".
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hachi8"Dinner is being ready."If you feel like using the present continuous tense and want to give the listener the idea that the action is in progress, use a verb, for eg: 'cook' or 'prepare'.
hachi8Then I have one more question. Is "Dinner is going to be ready (soon)." within the range of natural English?"Dinner is going to be ready soon" is fine.