Borscht as is known all throughout most of the ex-USSR and the world is Ukrainian. The red beet version, that is. The more general term roots in Poland where (even much earlier) it meant a soup with hogweed (hence the name), then sour soups in general.

Kvass should be safe though, it's a generally east-Slavic thing, basically low to negligible ABV sour beer from rye. And yes that's exactly what Russians are drinking anyways, the cola craze after the fall of the USSR was short-lived.

And just to add injury to injury: Pelmeni aren't Russian, either. The name is Permic pointing straight to (near) Siberia, it's thought that Mongols brought the general idea of dumplings from China there on their adventures which also explains the pepper. OTOH that means that the stuff you see in supermarkets, frozen bags of them to throw in boiling water, is 110% traditional (modulo the plastic).