Here is what it could look like with all comments taken into account:

I leave only the 'Upload to Commons' button, with the warning message if the tickbox 'suggest machine translation' is ticked
If machine translation suggested, I change the background color + font to italic and bold to mark the difference
if an existing translation exists, it is loaded in the box.

1. checking the "Suggest machine translation": Yes, text field will be filled with machine translation. Will add a spinner / visible loading state when this happens.
2. difference between "Accept Suggestions" and "Submit": i want to make sure the acceptance of machine translation is intentional to ensure the quality. I can remove the "accept suggestions" button and add a warning text box instead.
3. I am not sure i understand the use case when a manual translation would already be in the box: is it if a user starts translating manually then ticks the 'suggest machine translation' ?

in any case, i propose to
3.a add the machine translation only when the text box is empty (no previous manual addition)
3.b change the background of machine translated box to light green to clearly indicate the boxes which are suggestions from the service
attaching a new mockup

I m trying to help with this task. here is a proposal
UI: mockup:

1. machine translation is optional with a tick box, must be actioned by user
2. if machine translation is selected, for each segments, propose a suggestion (API call)

3.user must intentionally accept translation before submitting