Just came to say one thing.
As I said when we got the spoiler, the dialogue says 'pair of wings/both wings'. It didn't say anything about "One of the wings", the translator just did his job and translated what the chapter said without using what Oda said before in the SBS. If you go to a japanese dictionary and search for the word, the only thing you will get is "pair of wings", etc. Nothing about one of the two.
Just don't cause trouble to the translators and just send a letter to Oda for the next SBS if you are that curious.
For your information, the Spanish translation translates the sentence as one of the wings, but as i said, the dialogue in the chapter doesn't say that.