Recently I read an article (Prestigious journals make it hard for scientists who don't speak English to get published, study finds) stating that there appears to be minimal effort in a wide swath of (bioscience at least) journals to be more inclusive on the language front. Essentially, publishing in scientific journals, most of which are in English, is much harder for non-native speakers than for native speakers.
Shortly after this, I just happened to receive a request to referee a manuscript which is clearly written by non-native speakers. I like the content of the manuscript, and I plan to expend significant effort to address the specific grammatical issues within the paper.
The question is: how should I go about writing the comments in the report tactfully, which is to say that the authors would not take offence, and also find the comments helpful?
This issue is a particularly sensitive one to me personally because I am a Chinese-Canadian who did all of his education in Canada from secondary school onwards. Nevertheless, I have received a profoundly rude referee report that baselessly attacked the grammar and general writing of a manuscript of mine, while suggesting "the authors should consult a native English speaker before submitting the manuscript again" (my coauthor is ethnically Japanese but also did all of his education in Canada). I have to emphasize that the report was very badly written and was riddled with grammatical errors itself. Because of this, I have vowed to never submit a paper to this particular journal again (though I did not bring this issue with the editors).
I would not want to make the authors of this paper feel what I felt when I received that particular report. So how do I address these issues tactfully in a useful manner?