BCG ambassadors from Tokyo office told me a few years ago that most case work is in Japanese. Therefore I expect the interviews to be mainly in Japanese with portions of it in English to test your fluency.

As far as preparation go: with only two weeks available and starting from scratch I would suggest the following:

1. Buy/download one (max two) reference book/reference online course. Case in point, Victor Chang, P4S are all valid alternatives. This will serve as the main reference throughout the two weeks. Would try to have a quick read/go through of theoretical part within next 48 hours and then go back to it after each mock interview
2. Start mock casing immediately and target to do at least 14 (days) * 2 (average mock cases per day) = ~30 mock cases before interview day. And by immediately I mean at the latest after reading of the theory portion of books is done, but can start do mock cases concurrently. I would suggest the following mix of interviewers:
   1. Preplounge experts, possibly former interviewers for BCG (full disclosure here - I am one but do not speak Japanese). Use them throughout preparation but especially at beginning and at the end of it - this is because they have interviewed before and can give the most targeted feedback based on scoring criteria of BCG uses
   2. Preplounge community members between days 3 and 12
   3. BCG Tokio consultants between day 10 and 14 (one or two cases max, see below). This way they can assess preparation when it is at ~80% of potential and answer questions on interview specifics (e.g. language in which is held mentioned above). This depends on two factors:
      1. Whether interview was arranged through a personal contact with Tokio office. Not important whether the contact is part of recruiting or consulting staff, it can be asked to organize to either
      2. Whether this is a common practice in Japan - in US and Europe is a very common practice to help candidates by having someone from office investing time to give them one, max two cases before their interview day. Either way do not be afraid to ask, it won't affect your chances in any way, shape or form, just follow common sense (be polite, do not abuse of availability and limit to one or two cases unless offered)

Another thing I would very strongly recommend - depending on office culture, I have seen this working in US and Europe - is to ask whether is possible to move the interview back a couple of weeks to give more time to prepare. In US and Europe when a candidate is given a mock interview and we realize s/he is not ready but we can clearly see potential we ask recruiting if it's possible to move to next slot of interviews to give him/her a fair chance.

Two weeks is a very short time and I believe it would be a fair ask (again, depending on office culture). Office is aware that case interviews are not a natural skill and practice is needed to refine/polish delivery of them.

Best of luck, and hope the above helps,

Andrea