Croatian

Croatian is the most prominent of the South Slavic languages — about 5 million native speakers in Croatia and Bosnia, mutually intelligible with Bosnian, Serbian and Montenegrin (the so-called BCMS continuum). It's the language of Dubrovnik's Renaissance literature, of a Mediterranean coastline that gets more Instagram tags every summer, and of a small but punchy literary scene from Krleža to Ugrešić.

For English speakers, Croatian is genuinely difficult but punches well above its weight in usefulness. The seven cases and three genders are the main hurdles, plus the famous Slavic verbal aspect (perfective vs imperfective). On the plus side, the alphabet is strictly phonetic — you can read anything you can pronounce — and the language unlocks the entire Western Balkans. A Croatian speaker can travel from Slovenia to Macedonia and be understood almost everywhere.