So I'm sure I've read a thread on this somewhere on this site, or it came up in a thread somewhere but I can't find it.

When setting a story in a country where the people do not speak English, when is it appropriate to use non-English words? Some stories just throw (especially Japanese) foreign words around for no reason at all, though some concepts really don't translate.

So what does everyone think? When do you use foreign language words, if at all?

For the record, I'm working on a China story (or trying to) and dealing with family names is a pain, e.g. there is no word for just "grandmother", the terms identify paternal or maternal in the word. Chinese is ridiculously convoluted when it comes to family names. Translate or keep in Chinese with explanations? What is worth explaining (my father's third uncle who is younger than his father)?