Does anyone know whether Japan is like the US and China in that you must only talk about a great candidate's strengths, or like the UK where letters are dismissed unless you also talk about weaknesses?
For Japan, you must focus on strengths, just like the US and China. If there are any weaknesses, rather than focusing on the weaknesses, focus on how they were overcome, without dwelling on it too much. Keep the weaknesses to a minimum, and be concise. Enthusiasm is good and advisable, but it is fine to tone it down slightly, when compared to the "ecstatic" US style. Note that this is generally the case for japanese public universities, but private universities historically established by foreign missions with strong ties to the UK might prefer the UK style. Since I'm not aware of the situation in these private institutions, I hope other SE users with more experience can pitch in.