Partly with the point of summarizing other answers/remarks: there are several points mixed together here. There is the issue of recognizing "prior art" (whether or not one's present work depends upon it). There is the issue of recognizing "necessary background" (often logically prior to the work in question). There is the issue of "competing, perhaps incomplete work" (to be civil, as well as being a mensch and informing readers about it). There is "compliance-necessitated reference", as in "icons" which, if not cited, will make the skeptical quasi-expert reader doubt the competence of the author.
Of course, there have been times in which otherwise-intelligent people did not realize they should write in English. (Sorry, part of my reason for making an "answer" was to publicly form that sentence. I did grow up in midwestern, mid-20th-century U.S., and was led to believe that people who didn't speak English were just being obstructionist jerks, since obviously English was the universal natural language... whew!)
Another perversity: more than one historically-famous mathematician has let slip to me in conversation that they take the viewpoint that not looking at someone else's paper relieves them from any obligation to cite it [even if it is prior work]. Not to my taste.
More perverse than pretending to ignore others' work is the actualy benefit their work may give you/one, even, or perhaps especially, if one hasn't assimilated it yet. As a historical example, it confused me for a decade or two that Siegel and Harish-Chandra (both at IAS) had apparently not communicated... ever?... so that the "holomorphic discrete series", visible in Siegel's and H. Braun's work in the late 1930's, were not visible... and, then, contrariwise, Shimura and other "modern" automorphic forms people seemed functionally oblivious to H-C's work 10-15 years earlier.
Must/should one personally certify anything cited? Well, sure, obviously, this would be desirable. Also, not feasible. Of course, if you're counting on the correctness of an obscure paper, you are on thin ice. If you're counting on correctness of an already-often-cited paper, then you are in better shape. No mystery here. Refereeing does not assure correctness, just makes it a tad more likely. If you claim to prove something scandalous, people will revisit all those innocent-seeming papers you cite. "Just the obvious." (But, contrary to the professional pose that ... oh, ... published papers are first-order predicate-logic ... correct? A needless conceit, of course, and we should not slip into it.)
The editorial pressures are corruptive. Yes. The professional competitive pressures are potentially corruptive, yes. But I think if we are honest with ourselves we can see what acknowledgements we should make. Prior work, even if we don't use it. Even if we are competitors. Give the reader (supposing they care!) a guide about how to arrive ... here.
Depending on how one understands the word, I think that, happily (to me) "honesty" is a good guide.
Oop, "was it in English?" What? Um... My own discussion, and all others, indicate that this cannot possibly be a legitimate issue. Write to the editor. But it is a "stimulating" question. :)