Or rusalka.... In English you could either just keep rusalka as in ,,The rusalka came from the water" or translate it as ,,water nymph" because that's ultimately what a rusalka is. Same with vila. You could keep vila (,,At noon in summer, vilas appear in the fields") or translate vila as ,,sun demon" or ,,wheat witch" or something. Do you understand what I mean? In original it's Wiedzmin, in English Witcher, in some other Slavic languages it's Vedmak. Still it's the same creature, but if You google Wiedzmin you'll get Polish result, if you google Witcher you'll get just the books :D