Despite us using the language fallback chain, and despite the fact that "this should never be the case", there still can be properties (or any entity) which do not have any labels whatsoever. Currently ForeignApiEntityLabelRepository (made during T231465) does not handle this gracefully, but should.
I think (technically) throwing a dedicated exception and (ux-wise) showing the property id as a consequence should be acceptable ways of handling that.