For North Indians there is no problem related to language (except the fact that translation of CSE pre-paper from English to Hindi is little tough & sometimes they use unconventional words but it can be ignored on grounds that translation always destroys the creativity & it may always not convey exactly the same thing.)
But the major language problem lies with South Indians. Prelims paper is available only in English & Hindi so if you are from North India you can look up questions in both the language. While the same advantage is not available to candidates from other regions.
The only option for South Indians is English as they are less likely to be aware of Hindi.
In the classroom I find many South Indian students who are unable to take dictation of class notes in English. While I don’t find this problem among the North-Indians.
What one more thing is clear is that South Indians would also be finding it difficult to follow up the class sessions.
But there is no such problem in Mains. As one can choose even regional language to write.
So, the little problem is till you clear the prelims.
PS- it is not a gigantic problem but sometimes may act as a deterrent to pace up the studies or even to think of civil services as a career option. But with little patience & hard-work the problems associated due to language barriers could be overcome (may be 3-4 months to get comfortable with English).
With Great Love,
Er. Amit Yadav