I really enjoy the Oxford Dictionary of Quotations. It often has quotations in their original language - but it has Marx in English. Marx lived in London for much of his life, so I would assume that at least part of his writing was in English. Here's the quotation on religion from Oxford Dictionary of Quotations:
Religion is the sigh of the oppressed creature, the heard of a heartless world...it is the opium of the people. A Contribution to the Critique of Hegl's Philosophy of Right (1843-1844)
I think this was written before Marx moved to London, so I won't guess what was the original language, or whether Marx or somebody else did the translation.
Whatever the case, I think that "opium" and "opiate" would have exactly the same meaning in this context.
Also note this quote from English writer and clergyman Charles Kingsley (1819-1875):
We have used the Bible as if it was a constable's handbook — an opium-dose for keeping beasts of burden patient while they are being overloaded. Letters to the Chartists, No. 2