In Japan, the official average body temperature is 36.0 °C. Why so different from that of Europe?
Factors that can lower body temperature:


low environment temperature
low metabolic rate
hypothyroidism


I couldn't find any relevant study addressing the Japanese people situation, but:


  There is significant variation in metabolic rate in humans, independent of differences in body size, body composition, age, and gender. Although it has been generally held that the normal human "set-point" body temperature is 37 degrees C, these interindividual variations in metabolic rate also suggest possible variations in body temperature [1].


And here's a factor that increases body temperature:


  The present experiment investigated the effects of skin pressure by foundation garments (girdle and brassiere) on the circadian rhythms of core temperature and salivary melatonin. [...] The results suggest that skin pressure by clothing could markedly suppress the nocturnal elevation of salivary melatonin, resulting in an increase of rectal temperature [2].


Whether there are differences between Japanese and other people in metabolic rate and /or clothing behavior remains unclear. 



References:


Rising R, Keys A, Ravussin E, Bogardus C. Concomitant interindividual variation in body temperature and metabolic rate. Am. J. Physiol. 1992 Oct;263(4 Pt 1):E730-4. PubMed PMID: 1415692. 
Lee YA, Hyun KJ, Tokura H. The effects of skin pressure by clothing on circadian rhythms of core temperature and salivary melatonin. Chronobiol. Int. 2000 Nov;17(6):783-93. PubMed PMID: 11128295.