I was refered by my friend who is working in Rakuten Tokyo. After two weeks of application, I received an email from Global HR for two interviews. The interviews were scheduled on the same day through Skype since I am at abroad. I was interviewed by the assistant manager and managers of the department. The first interview was mostly in English and some Japanese. The second interview was totally in Japanese. The interviewers were very nice and friendly.
The questions were mostly common questions such as:
1. why you want to work in Japan?
2. why rakuten?
3. did you read mikitani's book
4. could you accept working OT daily?
5. what languages that you can speck? etc.
They also explained that Rakuten is still a Japanese culture company although they implemented English as the official business language. I think they tried to find someone who can accept the Japanese working culture with little culture shock.