I have seen books where the author prefaces the book by saying it's a translation of some other-worldly book, and then goes on to use real-world languages as a stand-in for the in-world languages.
Tolkien did this to a minor degree when he used some more archaic English words for the Rohirrim, whose language was meant to be like an older form of the language everyone else spoke. Thus they use thee and thou and such while everyone else uses you.
However, as a reader I feel that it's always a little odd that the people in the book speak English and in a fantasy setting I'd be inclined to strip out every possible linguistic connection to English. So nothing in-story would depend on features of English, such as its facility for acronyms, or the etymology of its words, or its relation to other languages. English and Russian have some cognate words due to their historical development. Would there be any reason for the witches to have the same cognates as the other people in your story?
Ideally, if the readers are meant to understand the witches' speech, you'd write it in English and just tell the reader if they're speaking some other language, or typeset it differently (put it in square brackets, or italics, or something). Since you're contemplating Russian, I assume you want most readers to be unable to read it? In which case a made-up language would be more realistic, albeit potentially much harder.