Note, though, that you basically need to achieve native-like levels of proficiency in order to use one Slavic language to understand another. So you may well never be able to collect this return on the investment at all. My Russian isn't anywhere near this level and, living in Central Europe, the only use I ever make of it is talking to Russians. Signage in Slavic-speaking countries becomes somewhat comprehensible, but that's about it otherwise. Russian does, of course, allow you to communicate in Central Asia, the Caucasus, and the Baltic countries, which is nice if you want that, but also likely to be irrelevant for most people.