You don't say (here) what field you're in. I suppose that by doing this you intend to invite answers from different fields.
In my own field of mathematics, it would be quite strange for someone to be added as a coauthor primarily for their editing work.
Nevertheless I had a situation recently where I did substantially edit and rewrite a paper in which I was not an author. (I think this is already a bit unusual, but it's hard to know for sure.) In addition to the writing I included a small amount of content: "minor scientific contributions" covers it rather well, actually. Towards the end of the process I was offered coauthorship on the paper. I appreciated the offer but turned it down immediately: though I had contributed to the writing of the paper and contributed some mathematical content, the amount of mathematical content I had contributed was much less than that of the named authors. My perspective was, honestly, that they had already done the work but were having trouble writing it up in a way that would make it publishable in a good journal in a reasonable amount of time. To me, a good rule of thumb is that if subtracting your contribution would result in a paper which (i) still exists and (ii) could -- with additional routine work -- be submitted to the same journal, then your contribution was not sufficiently substantial to warrant coauthorship. In the case at hand, a description my contributions to the paper appears in the acknowledgments...written by me!
There is another way to look at it that in my case made me even more convinced that I did the right thing. When contemplating adding an author, ask yourself what that person would gain by being added versus what the other authors would lose. In my case I would gain at most one more publication -- in fact, the third of a series in which I was a coauthor (the most senior one) on the first two. The two student coauthors would lose the prestige of having written a nice paper which does not have a faculty coauthor....as they deserve, because they did more than 95% of the mathematics of the paper on their own, without any guidance or direction from me. Adding myself as a coauthor would be undermining my own future plans, as I have and will again in the future talk about this work when recommending these students. But even if I was just a postdoc or a more senior grad student, fundamentally speaking how much credit can I get by being an author of a paper when as soon as anyone asks me about it I will feel honorbound to describe the minimal nature of my contributions?
I hope that by the end I have waded back into a point which is relevant to your question. Academics should not be in the business of maximizing the number of papers which appear with our name on them. I don't know of any academic field where this is really the route to substantial academic success: you get hired for the actual strength of your work, not the number of your papers. Adding yourself as an author to a paper that you mostly just copyedited and then made some minor comments from a position of lesser insight/expertise than some of the named authors: in so doing you're not actually adding strength to you research program, are you? Anyone who might have been impressed with the paper is going to be distinctly disappointed when they learn what you actually did, right? I know that in some fields (much more than mine...) the quantitative standards for publication are very high: in some branches of engineering and the sciences you most certainly want to ensure that you are writing a lot of papers rapidly. But then you want to really be involved in the work of those papers, right? We've seen on this site how easy it is to publish papers in the absence of actual academic content. It's too easy to be a plausible route to legitimate academic success in most parts of the world.
As I mentioned in a comment, you said that the one coauthor who is a native English speaker is the most junior author on the paper (a master's student). Well, that confluence of lightness of intellectual contribution and superior skill in this other domain makes that student the perfect person to do the copyediting, it seems to me. If their understanding is so limited that they can't even be trusted to edit the paper for non-content related issues, then I am worried about their being listed as an author at all: what could they have done?