The way each restaurant is run is different, so I recommend “When in Rome, do as the Romans.” Even if you are a chef with decades of experience, be prepared to start at the bottom. Until you know well the workings of the shop, there are only a few jobs that you can do.
You may be assigned to simple jobs such as washing dishes, putting out tea, cleaning and so on. This may also be true for overseas restaurants, but in Japan working in a restaurant is hard work. Besides the hard work, not understanding the language will be tough.
Although it’s a good opportunity to learn all kinds of cooking knowledge and techniques on-site, be aware that it might be a much harder job than you can imagine.