In some cases, one mentions the relevant paper, stating explicitly that they could not read it for language reason, e.g. "related results where obtained by Author A in paper P [reference], which we were not able to consult since the article is only available in Russian". That last part can sometimes be omitted, if the knowledge that you did not read the paper itself (but, say, only a review or a translated abstract) is not useful to your reader.
This, however, cannot apply to papers you build on in a way that needs you to make sure the cited paper is correct, or in a way that needs precise statements to be understood finely. If there exist (precise) reports or survey on this research, you can use the secondary source (and mention the primary source, but making clear which one you rely on and read) or, ultimately, a translation service (but I don't know where you can find one which can be expected to be reliable for highly technical papers).