Her American accent will disappear quickly.
Studies have constantly shown that little kids can learn multiple languages simultaneously, with little effort. It has to do how the brain is wired at an early age (seriously), and this is a perfect time for her to learn English, Japanese and Chinese…and even throw in another one or two languages if she seems to like it.
I had a friend in Berlin whose daughter needed to be fluent in German and English before entering school (at age 5). It was a private school. He was American and only spoke to his daughter in English - his wife was German and only spoke German to her. By age 4 that girl spoke both fluently - she considered one “Daddy’s language” and the other “Mommy’s language” and never had a problem flipping back and forth and keeping them separate. Perfect accent in both as well.
I worked briefly at a hotel in Germany and once had elderly Italians show up with their granddaughter. The girl was about 6 and did all the translations - seems dad was American, and uncle was German and mother was Italian.
At any rate, the little girl spoke fluently and did all the translations when booking the room and asking for directions and extras for the room. Quite amazing to watch. Spoke both German and English equally well, and then rattled it back in Italian.
Sadly, this talent to learn languages easily gets more difficult as we age.