Hello,


If you're enquiring this question in terms of the medium of the exam, then having English helps to a large extent as most importantly the study materials are readily available in this language though many students too opt for hindi medium and qualify as well, but somewhere the availability of study materials can be a bit issue here in some areas, but in general having a decent understanding of English is enough for UPSC, unless you have selected english literature as optional you don't have to study in depth about this language, hence have a good command over the language grammatically is enough, you can write general studies, optional and essay in simple language, and still you can fetch good marks.