I've seen this question on suggesting someone get a native speaker to edit and check. But my situation is rather different.
Several colleagues and I wrote an article for a special volume of a journal. The initial review was R&R with some pretty severe changes requested. We've made those changes. After that we received an odd e-mail from the editor for the special volume (via her assistant -- but via subsequent e-mail confirmed to be under her direction):
We have carefully read it over, and in order to move forward, we highly recommend that you kindly consider seeking out academic editorial services in order to meet ... standards.
Then later in the message:
We recommend that authors have their manuscripts checked by an English language native speaker before final approval of their submission; this will ensure that submissions are judged at peer review exclusively on academic merit.
The publisher and journal are not known for being predatory.
As a native English speaker, I was rather surprised that the journal said this. So I reread the article and there aren't any gaping grammatical or structural issues (I actually edit papers for others who are non-native speakers in addition to my own publishing, so I'm rather accustomed to the sorts of mistakes they make).
What is a good way to respond to this request?
Edit to make clearer, I absolutely agree that part of what one should do when told
find an editor is to find and get some objective checks done on the writing itself (non-author parties and other forms of third parties competent to check). My question is assuming you've cleared that hurdle and have received this sort of comment.
While one should avoid antagonizing editors where possible, I think it would be reasonable for you and your coauthors to respond with a matter-of-fact email that includes the following points:
(1) one of the authors, namely yourself, is a native English speaker;
(2) said author has re-read the revised manuscript, and did not find obvious defects in the grammar or idiom;
(3) it would be greatly appreciated if the editors or the referees could point out the exact places in the revised manuscript, where it is felt that further correction is needed to meet the desired standards.
If they are being picky in good faith, point (3) should not cause a problem. If they stall or ignore point (3), then at this point it might be worth asking other people in your field or related ones if they have had similar experiences with this journal or its editors.