What I found is once you learn a Slavic language, like Polish or Russian, it’s fairly easy to pick up the other ones, like Czech, Slovak, Serbo-Croat, etc.
That goes for reading and listening knowledge, but I wonder if all those words in one's head can make it hard to speak any language correctly.
Italians and Spaniards can generally communicate with each other by each speaking their own language (Come to think of it, it's getting that way in the US with English and Spanish, languages that aren't anywhere near as closely related). French is more different. It can often help you to guess a Spanish word, but it's not always the right Spanish word (or even a real Spanish word).
If you know English and German, Dutch shouldn't be that hard, and German helps with the Scandinavian languages, which (linguist John McWhorter says, maybe half joking) are all basically really one language anyway. I think in those Scandinavian cop shows the Swedes and Danes and Norwegians understand each other without having to learn or speak the others' language.
When I was 12, I asked my Grandpa if he would teach me Czech. He grumbled a little at me, then said something in Czech. I repeated it a few times, and he corrected me when I said it wrong. Then I asked him what it meant. The answer? “Go jump in a lake.”
I didn’t understand that was an idiom till I asked my Mom about it. Silly kid! I didn’t know it at the time, but I must’ve been such a pest back then. Of course, you know I had asked him while he was busy working on a project. Love that man!