At least Swedes seem to be ridiculously good at speaking German after having just a few years of German in school. Language learning has lots to do with similarity.

The other important point is how much you use a language. Americans simply don't have contact with languages other than English most of the time. You also see this effect in Germany where the English level is fairly low for speakers of a language that is so similar to English. And this because tv and movies get dubbed in Germany.