I'm Polish, I've never learnt any foreign Slavic languages, but I can mostly understand Ukrainian, Belarusian and Slovak from passive exposure on a few trips.

The experience is weird - when I first went to Ukraine I couldn't understand almost anything. After a few days it suddenly "clicked" and I realized how the most common sounds and word endings relate between Polish and Ukrainian - and since that moment I basically got all the words with the same roots for free (which is like half the language). It also gave me Belarusian as a side effect :)

Of course it's not actual language speaking, I'm just understanding every other word and connecting the dots.

With Slovak it was even quicker, but somehow it hasn't given me Czech - their pronuciation is just too weird, despite the fact it's almost the same in writing :).

I'm not sure Interslavic provides much value. For me it's basically Slovak, so if you come from East Slavic language you'd probably get the same benefit spending that time learning some Slovak, and you'd then know some actual language instead of an artificial one.

There’s a surprising amount of vocabulary difference across Czech and Slovak, in addition to the pronunciation differences you mention. Pre-split everyone on both sides grew up hearing bilingual broadcasts, so they picked up the differences ambiently. I’ve heard that it’s a lot less of a given whether the younger generations presumptively understand each other these days. But I’ve also seen evidence that many still choose to engage with people / content / opportunities on the other side enough to get to solid working familiarity anyway.

This comes from limited first-hand experience and more extensive second-hand cross-generational experience. Take it as you will.

Slovak children usually grow up with Czech narrated cartoons, so they are able to understand Czech more easily. I heard that Czech children does not receive this language training for Slovak, so they have a harder time understanding Slovak language. I never "learned" Czech in school but I watched a lot of cartoons as a child (born '93) and read books in Czech so I have no problem understanding Czech language as a Slovak. I have a hard time understanding Polish though, never clicked for me.

I can confirm this is true. Czech republic is cca 2x the population size of Slovakia and its historically more developed part, so during one state union a lot of media were in czech language and it became our second language without thinking about it. Also Czechs did get a decent exposure to slovakian language.

But if there is no exposure, its becomes visible how grammar is very similar, but most words are just a bit different (very few are completely different), and pronunciation varies so much across whole region (even within given country) that its not easy or even possible to understand each other out of blue, without prior exposure.

I got some exposure to Polish TV during 80s, since commies couldn't put together more than 2-3 channels on TV and those were anyway pretty bland. I can cca understand it, but can't say a single sentence well enough. If I read polish text, I have to read it loud in my head and then I grok it easily, otherwise too much 'cz', 'w', words are too long etc and I lose meaning very quickly.

But in general Polish is a bit further away from either Slovak or Czech languages. We were and still are literal brothers (CZ and SK), extremely similar in so many regards, still see no good rational reason why we split up (of course I know real reasons, but those are nasty as are the people responsible for the split).

The US state department has some estimates of time to become proficient in target languages for English speakers. Germanic and Latin based languages take the least amount of effort:

https://www.state.gov/foreign-language-training/

Interesting link. Some of those estimates are very, very optimistic though...

It helps that staff are participating in these programs full-time, no other duties required.

A story I've heard first hand is that after the program, department staff can discuss diplomacy in the target language, but struggled to order a coffee!