We don't know this.
Indeed we don't... sorry 'bout that. It is fixed.
Hmm... so it seems that the consensus is that Kaworu's been pretty isolated, except whatever contact was necessary, which came from Keel/his handlers. It also seems, according to pwhodges (and this makes sense to me) that his knowledge of different things (like the Japanese language, and Ode to Joy) could have been imprinted on to him through some... artificial means.
I wonder: Maybe the imprinting process worked by transfer of certain, long-term, almost innate parts (like language ability) of an individual Lilin's memories to the recipient. So, that perhaps there may have been some unwanted/unnecessary memories (like the Ode to Joy) that leaked in, because of the interconnectedness of human memories?