Can polish speaking people understand croatian?
Nope, even it’s Croatian is too a Slavic language, Pole won’t be able to understand it, except few words. People who were studying Slavic Philology such as myself will be able to recognize some basic forms and terms as some are similar (as all Slavic languages evolved from Old-Church Slavonic language). But as long as you never learned Croatian, you won’t be able to understand it ;).
