I've seen LLMs hallucinate nonexistent things in programming languages. It's hard to believe it won't do the same to human ones.
If a language model were asked what the word for "floppy disk" was in an extinct language and it invented a decent circumlocution, I don't think that would be a bad thing. People who are just engaging with the model as a way of connecting with their cultural heritage won't mind much if there is some creative application, and scholars are going to be aware of the limitations.
Again, the misapplication of language models as databases is why hallucinations are a problem. This use case isn't treating the model as a database, it's treating the model as a model, so the consequences for hallucination are much smaller to the point of irrelevance.
An LLM should do fine with that since it's usually the foreign word spelled in a way that makes sense in that language. I'm more curious about the inverse though. It's sometimes quite difficult to explain the meaning of a word in a language that does not have an equivalent, be it because it has a ton of different meanings or because it's some very specific action/object.