I'm not too surprised that Americans and Brits don't speak other languages. I doubt people here (Denmark) would speak more than one language if Danish were the lingua franca of international commerce. As it is, the vast majority of Danes are fluent in two languages, English and Danish, because they are both necessities to function in modern Denmark (Danish is the national language, and English is the commercial/business language and language of travel).
People do learn other languages, but at about the level Americans learn other languages: a few years in school, rarely able to carry on a conversation. Older people are more likely to be able to carry on a German conversation, but among younger people it's uncommon. If anything there's increasingly a little bit of nationalist suspicion about people who speak third languages, especially third languages natively. It's normal to be fluent in English and Danish, but if you're raised speaking another language at home, that smells of the dreaded "multiculturalism", and people may actually criticize you for raising your kids speaking a foreign language at home instead of fully assimilating into Danish culture and raising them in Danish. (Goes doubly if the foreign language is Arabic or Turkish.)
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.
People do learn other languages, but at about the level Americans learn other languages: a few years in school, rarely able to carry on a conversation. Older people are more likely to be able to carry on a German conversation, but among younger people it's uncommon. If anything there's increasingly a little bit of nationalist suspicion about people who speak third languages, especially third languages natively. It's normal to be fluent in English and Danish, but if you're raised speaking another language at home, that smells of the dreaded "multiculturalism", and people may actually criticize you for raising your kids speaking a foreign language at home instead of fully assimilating into Danish culture and raising them in Danish. (Goes doubly if the foreign language is Arabic or Turkish.)