There's a simple answer in this particular case - transliteration of «спутник» is "sputnick", which an English speaker would naturally pronounce "SPUHT-nick".
It's less clear in cases where the word changes in ways that can't be explained away by the limitations of transliteration or the sounds available in the English language. I'm sure there are hundreds of examples of this, but I can't think of any at the moment because I'm too tired and should go to sleep instead of posting on this forum![]()