I'm sure everything will be fine for you, friends of mine had a European student with them for some time. They had no children, but it wasn't anything like the student became part of the family, more like it was a student base - to sleep & store her stuff.
As a student myself I had foreign friends who also only used the bedroom of the house with a few hello's.
You say above that the Japanese girl requires Bed, Breakfast & evening meal, sometimes this is all they want as they are very busy.
As long as she has her space & feels welcome you will be the perfect hosts. Your younger daughter maybe expecting a 'sister' figure and find she may be disappointed.
I'm not sure if this is still the case, but the Far East don't have dairy in their diet, (only recently introduced) and many are lactose intollorant. So milk, cheese, yoghurt ect is a no no until you know where she is on that. Stock up on soya this & soya that.
You sound like the perfect hosts and I'm sure she'll be the perfect guest.