somebody mentioned that ukrainian and belarusian are kind of a dialect continuum between russian and polish or sth like that... are ukrainian and belarusian that similar? would one be able to understand the other fairly easily if one knows one of them? Most of the other slavlangs aside from serbian don't really interest me much and I have mixed feelings about polish so as far as a secondary slavic lang to 'look into' right now belarusian and ukrainian are looking the best for me...
the idea of macedonian and bulgarian losing the use of the cases kinda feels, idk, boring, cheap, lame, non-slavic, somewhere along those lines...
Russian Ukrainian and Belarusian relate to each other pretty much the same as Danish Swedish and Norwegian do - there are phonological, morphological and syntactical differences, as well as vocabulary differences, but they do not generally prevent mutual understanding (this might be less true for Russians who are less likely to be exposed to other 2 languages).
would one be able to understand the other fairly easily if one knows one of them?
If one is a foreigner who reached a good overall level in one the languages, then with just a few lessons introducing them basical phonology rules and vocabulary differences, it should be fairly easy.
somebody mentioned that ukrainian and belarusian are kind of a dialect continuum between russian and polish or sth like that... are ukrainian and belarusian that similar?
The fact that they fill the continuum gap between Russian and Polish does not imply per se that they must be close, does it (they might be close on X-axis but distant on Y-axis).
Belarusian might be harder to start with since many resources for it assume that you already have at least some knowledge of Russian. As far as I know Ukrainian is more accessible without any prior knowledge of any Slavic language.