Is it appropriate to cite a paper in a language I don't understand?
I think the fact that the paper is in a language you don't understand is somewhat of a red herring. Sure, it will affect the specific steps you'd take to try to understand the paper, but the ethics of "to cite, or not to cite" it is no different from a paper you don't understand written in a language you speak. You're generally supposed to provide a good-faith overview of the literature, with citations as appropriate. The fact that one paper is significantly harder to understand doesn't really change that.

My view is that it behooves you to, as far as possible*, and regardless of language, at least understand at a high level what the full papers you cite contain, and the parts of papers you use on a detailed level. Google Translate will be good enough in some cases, but for other cases it's better used as a starting point. If you find yourself doubting the machine translation it's certainly best to use it as a starting point. You might want to try to clarify uncertain points with the authors, ask a colleague (who might speak the language) for help, get a professional translation of parts of the paper, or use a secondary citation.

*In some fields they even study ancient languages to understand original texts. That might be overkill...