I'm sure most people don't know that proofreading is an option when selecting Translate. After all, proofreading isn't translation. I think it should be an option on its own in the menu. It could open the same tab in the Translate interface, but identifying it clearly might help with discovery.
This said, I don't think proofreading is up to the standards that it should be. I'm a professional writer, and it gets a lot wrong. So maybe fix it before highlighting it.
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