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...

You cite earlier works either because you directly built on them, or to provide the background against which you are working, and to position your new contribution in the larger context of existing research.

You should be able to judge whether a given foreign language paper warrants inclusion for the second reason based on an English language abstract and/or Google translate. If it does, cite it.

Also: nothing keeps you from including a footnote, or a parenthetical disclaimer.

Foo and Bar (2019) discussed the ethnographic complexities of underwater basket weaving (as judging from the abstract; Foo & Bar [2019] is unfortunately in Amharic, which we do not speak).

Reiterating the main points (as I see it) of the other answers: if it's relevant, cite it... both to show your own awareness, and to acknowledge prior art, whether or not it exactly impinged on your own work.

That is, whether or not an individual can fully vet a piece of work, through difficulty with the ambient language or whatever, acknowledgement of its existence is very important, I think. Citing things is different from endorsement, and is different from a claim that one has fully checked all details, or even read the whole thing through. Just be clear, in the citation, what use you made of it, or did not.

Once again, straightforward-ness, honesty, are better guides much of the time (too bad not always... I know...) than stylistic prescriptions.

As an example to perhaps not cite: if you happened to read arguably crack-pot documents on arXiv (for math, for example) purporting (implausibly) to do amazing things in your specialty, I think you can justify not citing... if the author has insufficient credibility, and no one else with any credibility has vouched for it. But, yes, what the heck is "credibility"? :)