[avenoir]

This is interesting. I'm native Russian speaker and outside of catching a few similar words here and there I think Polish sounds absolutely foreign to me. I think just about the only language that I can understand with some effort is Ukrainian.

[viridian]

Belorussian? IIRC Belorussian and Russian are the two most closely related languages that are treated as separate languages and not dialects.

[_Tev]

Closer than Czech vs Slovak? I think around 90% of Czech people understand 99% of what Slovaks say ... It's difficult to measure in opposite direction as its customary to watch Cz TV channels for Slovaks (at least from what I've seen / heard)

[nerfhammer]

I have heard Norwegian and Danish being described as such

[vidarh]

Modern Norwegian is a mix of Danish and dialects from the Norwegian countryside that has then been put through several rounds of reforms intended to bring them closer, which has to some extent made Norwegian more different to Danish, but yes, they're extremely similar. Even more so if you use conservative spellings of Norwegian. 50's or 60's newspaper articles from one of the conservative newspapers for example, might easily get confused with Danish by modern Norwegian speakers (but still be easily understandable).