Most modern Japanese homes have very plain wallpaper in either white or cream color. The game has those options in it. That bumpy wallpaper would work good too.
For the kitchen, the cheapest cabinet option looks a lot like something you see all over Japan in modern homes.
Actually, building a modern Japanese home wouldn't be hard. I just started building one (but my guy is still poor, so he's building it a little bit at a time.) Use simple wall and window options, simple painted walls in white or cream, the cheap cabinets. Nothing fancy. Some homes are using high ceilings and lots of woodwork, similar to a north woods resort. It's simple woodwork though, not something with elaborate carvings.
Japanese kitchens are not as fancy, or large, as American kitchens. Cabinetry should be very simple, with no fancy carvings or anything. The cheapest cabinet option is perfect. Most Japanese don't have ovens. Microwaves are becoming more popular though, so perhaps get a microwave without an oven. There is a growing trend nowadays for kitchens that either have a window connecting the kitchen with the living/dining room, or a counter separating the kitchen from it instead of a wall. This is easy to do with the cheap cabinet option also.
The entry way needs a place where people can take their shoes off. People don't wear shoes in modern Japanese homes either. That entry room area can be a little lower than the rest of the house. I still haven't figured out how to make it so that sims take their shoes off when they enter his house. My daughter is struggling with that too. There must be a way though, because some of the premade Japanese homes do this.
You can add a tatami room, but most rooms in a modern home would be western-style. I think most Japanese like to have at least one tatami room, but like western style for the other rooms. Put a kotatsu and nabe in it. It's good to have the tatami room near the kitchen, because many people use it as an alternative dining room. The tatami room is not a requirement for a modern house though. Some people don't have them, because they prefer western-style. (If you want a nice tatami room, download one from the gallery. It is a LOT of work to organize the tatami mats into a realistic layout, and so much easier to just download one that somebody else did the work on.) But most modern Japanese homes do have at least one tatami room. It's really nice if the tatami room has big shoji doors or sliding glass doors that look out onto a garden.
For lighting, use the simple (cheap) hanging lights. However, those round, white Norwegian (or Swedish ?) lights are also pretty popular in Japan, and one design option has a cherry blossom design on it. A crystal chandelier option is nice for a westernized dining room though.
I recommend a Japanese style bathroom. To do that, the bathroom is divided into 3 rooms. You enter the room that has the sink, mirror, cabinets, etc. That is connected to two small rooms, one for the toilet and one for the bathtub. This is important, because the Japanese traditional belief is that it's gross to put the toilet in the same room with the bathtub. Some cheap apartments have the toilet, sink, and bathtub in the same room. They call this "unit bath". But that's just to save money, for people who can't afford a home that has them separated. Your Japanese sims would much prefer to have them separated. If you have the laundry expansion, you can put the washing machine in the room with the sink.
In build mode you'll see a wall mounted shower (like they use in the onsen). Put one of those in the room that the bathtub is in. Your sims need to shower off before they get in the bathtub. The room for the bath should be all tile floors and walls, but that's not necessary for the toilet and sink rooms.
Most floors (besides the tatami rooms and bathroom) would be hardwood floors, or floors that look like hardwood floors. No fancy patterns though.
