It seems the editors would like the paper to be presentable in the journal.A correct language enhances the reliability of the results, and the ability to communicate the results to other scientists. Many journals today offer editing service to researchers to adjust their English level to that of the journal.
Because English mistakes are often more obvious than scientific mistakes, so they are easier to catch. To understand how or why a paper is based on poor science you usually need to read it in depth, and have an understanding of the topic. To see a paper is written in poor English usually only takes a cursory read and moderate command of the language.
Dear Ali, the English issue in the research paper makes me nervous. We all have so much to say in our native languages. Many times the research lose its value due to translation in English. And the editors says that the paper should be checked by a native English speaker. Where are we suppose to find one? We are all speaker of the English as a foreign language. What about the English native speaker who does not understand the subject matter? I am supporting that we should be aloud to write our papers in our native languages and there have to be international journals that will do this. I am so bothered with the demands of the editors. You have to spent a week to settle the paper according their famous parameters.
In my experience, it's really hard for an English speaker to review the scientific content of a paper with poor English, as the ideas are often not clearly communicated and the incorrect grammar is very distracting. As a voluntary reviewer, it is not my job to correct the English, it is the responsibility of the authors. The worst is when the co-authors are all native English speakers but have not bothered to help their non-English-speaking student correct the manuscript.