I think it would depend if you are doing a mainly quantitative study or not. If you are are mainly number crunching then I think you could get away with it. If it's qualitative, it will be a problem as translation costs are prohibitively high for most students. I know people who have got away with it by insisting their interviewees spoke English but then it's dubious how much you really get about what matters, and your available literature reduces substantially. I also think a four country case study approach is too many countries, so if you speak the languages of the others, why not just drop Hungary? Also although this is obvious, think through what research methods you could make a viable claim to use - obviously you couldn't do discourse analysis or ethnography if you don't have the language skills for example, but there are others.