Well, I would say -intimate knowledge !
Well (thanks for the links *grin*)
I would say knowledge, even intimate knowledge, isn't the problem here. One only needs to live between, immersed in the other culture or at least have a huge interest to learn.
Understanding is something else (hence my question)
In my opinon it needs at least a huge part of empathy and even similiarity (in outlook, values and attitude to life)
to understand the other.
For example even if one knows why the other culture did what she did in one event, in one time (or members of it), it doesn't mean that you yourself, with your roots in another culture, would act the same...
But to answer the question, yes it is possible for a foreigner to understand another culture...sometimes even better than his own if the adopted, even preferred, culture is much more fitting ones own values and attitudes..