Strategies for reviewing a paper that has non-English references
Evaluate the correctness and significance of the results as best you can.  On the topic of novelty, include in your referee report exactly what you wrote here:


  [This paper] quite heavily builds upon work that has only been published in Russian and Romanian, which are languages I (unfortunately) do not speak. Therefore it is quite difficult for me to assess how this new work builds upon this previous work.


Then let the editors decide.