So they might be untranslatable in certain contexts, for sure, and you have to scramble for alternatives which inevitably lose subtle shades of meaning.
That would be "difficult to translate", then, not "untranslatable", no? As for breaking the flow of the text, I'm pretty sure you're going to have to do that anyway translating Russian to English.
Example:
A Pole, a German and a Russian meet the devil. He locks each one of them in a cell, gives them two small marbles each and tells them they have a month to impress him with them, or he'll eat them otherwise. So he comes back after a month and sees that the German's cell is full of complicated mathematical formulas and one marble is perfectly balanced on top of the other. He nods his head and moves on to the Russian's cell to see him doing one-handed pushups while juggling the marbles with the other hand. The devil nods again and moves to the Pole's cell, only to see that he lost one marble and broke the other one.
Is it funny? Because it's pretty good in Polish and I don't know how to translate it withotu longwinded explanation.
The problem is that the word "zepsuć" used for "breaking the marble" means "to spoil" or "to make something nonfunctional without destroying it physically". In English "break" is overloaded and people assume other meaning.
So the Pole in the joke managed to make the marble stop doing what marbles do without damaging it.
There's an opposite self-stereotype, captured by the phrase "Polak potrafi" (a Pole can do it), which actually emphasizes resourcefulness as a national trait. Although it's often tainted with irony, or at least implies that the creativity employed isn't really by the books, etc.
Where would you draw the line? No culture has a concept which is literally unexplainable to any other.
Translators could potentially give meaning-for-meaning accurate translations if it were desirable, or possible, to give each word a paragraph of qualifiers and contexts. But translators are restricted to at least trying to preserve the structure of the translated work, and for words that means trying to pick a one- or two-word substitute.
Some words just don't have an accurate substitute. You can easily pick translations which are "not wrong", but none would be correct. What would you call those words, if not "untranslatable"?
But, I enjoy prozd's short sketch on "official subs vs fansubs" in this regard: https://www.youtube.com/watch?v=YvNxgHTWIlo (It demonstrates translation where an 'official' one is lossy, but the fansub tries to be more faithful).
Translators that work dubbing movies have harder constraints and seldom fail to make sense, only when there're wordplays or words that sound similar to others...
When watching dubbed things (which in Swedish means exclusively children's shows) I often hear things which make little sense until I suddenly realise there's an English sentence with similar structure that would make sense in this context.
edit: In other words, translations often fail to make sense when poorly done, and a lot of them are.
Sometimes I enable English subtitles keeping the dubbed Spanish audio and it's often well done. There is very talented people working on the translations, giving the right tone to conversations.
Compare "Fuck" said with mad, resigned and amazed voice. It's 3 different words in some languages.
Or compare a line "Our hero heard someone crying 'I loved her' from the crowd". In some languages depending what grammatical gender is the person shouting the word ending in 'love' is different.
English books often skip that information, so the translator has to deduce or ask the author who was shouting that line. In a movie you know because you can see that person or hear the voice.
This makes more sense to me because your relationship to your mother's brother is actually meaningfully different to your mother's sister's husband.
My German was minimal, but when a swagman got out of a semi that had just given him a lift, he said "Danke Comarade" and I realized that he said "Thanks mate" which has greater connotations and meaning than just a thank you.
All the characters in Winnie the Pooh represent a group of school boys. When Kanga shows up, it upends their world. In the canonical Russian translation and animation Owl is a fussy old lady. Which reduces all shades of meaning.
These are simple works of fiction, and already they are greatly reduced just by the singular gender choices made by translators. And there are dozens to hundreds of such choices per book.
Complex books like Too Like Lightning may be even untranslatable entirely because TLL rests on gender ambiguity. Russian has gender even in verbs, so you can see a character's gender even in simple phrases like "I did" which are ambiguous in English.
etc. etc. etc.