"You are shit" vs "You are the shit". Explaining "the shit" to someone who's fluent in English but not culturally fluent in American is almost impossible. There's a qualitative difference between "You are very good" or "You are the best" and "You are the shit". They're not exactly synonyms.
Another good example: Dude or Guy as used in Californian.
Or you skip all the context and just say it means "you're great", very easy translation.
"Eres el mejor" - I just translated it into Spanish
Different scenarios require different methods of translation, sometimes you'll want something literal, and sometimes you can just translate the intent behind the words. As long as you can do that, I would consider it "translatable".
A paragraph footnote might accurately convey the intended meaning but it won't evoke the same feelings. A good author or translator cares about both these things.
It's that emotional and cultural baggage layered on top of words that's hard to translate.
So for example in USA you can call someone "a benedict arnold". As an immigrant this means nothing to me. People tell me it means "bad". I understand the words, but there's no impact behind them because I lack the cultural background.
If somebody had first explained to you that Benedict Arnold was a well known traitor during the revolutionary war you'd have nearly the same context as your average American.
It's hardly what I would call untranslatable. I'm sure you could find some poor sod who the public education system failed so terribly that Benedict Arnold is as equally opaque in America.