It depends on the language and its relative family. Generally, a fluent English speaker would/might find it easier to learn a Germanic language than a language from another family like Slavic or Romance. The same goes with other languages; if some speaks Russian fluently, then it would be easier for that person to learn Ukrainian or Polish, because they are from the same family. The suffering starts when you branch outside of your language family. For instance, it would be super difficult for an English speaker to learn Azari but super easy for a Turkish speaker, and so on.
You don't need to look too far for an example. Here on Italki, almost every Russian teacher speaks Ukranian fluently and every native speaker of a Romance language knows at least one or two others..and the list goes on.