"He is the most peaceful."
Translation:Fe yw'r mwya heddychlon.
Ah, right - I see you have modified your question to clarify that. Because when we use a superlative (-est) form in Welsh, we put it into an emphatic shape of sentence, with the thing that is 'best' or whatever at the front of the sentence. In this case Fe needs to come first. It will be explained in the notes for the section, I think.
Edit - the explanation is currently in the notes for 'Comparing3'