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.
The one good thing about French is that a lot of words are similar to English, because actually a lot of English is based on French. But it’s a very softly spoken language that it’s difficult to pick up where one word ends and the next one begins.
I’ve heard Poles describe Czech as sounding like a baby speaking Polish.