The Office team in Microsoft Japan worked on docs.com and Office Lens while I was there.
You'll be assigned an onboarding project to start off with, and then move onto a main project to finish off the term with. If you finish your main project quickly, then you'll probably end up working on regular engineering tasks. The projects were in different fields, and a couple of them were quite interesting (involved NLP, OpenGL), but a lot of them were fairly standard web/mobile-development feature tasks. Obviously, these choices will vary each term.
Hours aren't particularly long, but they're pretty offset from the usual business hours (most people came in at 11 and left after 7). You're not pressured to commit to 11-7 at all though. Your mentor is mostly there to answer any questions you have, but you'll need to be fairly proactive in asking questions to get anything meaningful from them. I wasn't so proactive, and mostly ended up asking my mentor for help with Japanese paperwork.
Living in Japan is great, and there's a lot to see, do and eat. I highly recommend you take lots of trips on weekends and holidays!