The languages are different, even though both use Cyrillic alphabet.
There are some words that are the same, but, others that are completely different.
Therefore, if you don't wish to understand the other party, you can claim you don't.
If you wish to understand, but, don't....you simply keep trying and eventually you will hit upon a word that both understand.
Let me give you an example. When I visited Kharkiv, in very very eastern part of Ukraine (....really close to Russia), they spoke mostly Russian as most inhabitants were native Russians who settled in Ukraine. They were actually hostile to us, because we spoke Ukrainian. Mind you this was many years ago, and I hope it has changed.
At the hotel I approached the front desk and asked for a bottle of water in Ukrainian. The girl looked back at me and shook her head. I asked it in English. Nope, nothing. I asked it in Ukrainian using different words. Plyashka (bottle) vody, stakan (glass) vody...of water....nothing. I finally remembered one of the words my dad used when I was a kid and asked for a Bootilka vody...and voila! She finally "got" it. She knew all along what I wanted. Voda is Voda is water....she had to understand I wanted water, but, chose not to understand.
When I speak with our Russian parishioners, if I don't understand something, I always make a quizzical face and start squinting one eye...and they will try it another way...until I get what they are saying. I can usually get the gist, but, miss the finer details.
I'm slowly picking up more Russian, and it's becoming easier, but, if both parties are willing, there's really no issues in communicating. It's the will, that's the problem.