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"?