Making plans for your future is a great idea. The way to get there is by reaching short term goals, like getting great grades so you can get into college, then deciding your major. Who knows? By then you may have decided you have a totally new passion you feel the need to explore. Stay excited and open minded toward your future. I've had three separate careers and loved them all. And the part about being an American in London, let them judge you as the finest American they've ever met!
What's your citizenship?
If you don't hold an EU passport, the 'living in London' part of your plan is going to be incredibly difficult without skills and qualifications with which you can get a work visa. Think STEM careers and masters qualifications.
NB - London is a capital city, not a country.
You don't need to study Japanese to teach English in Japan. Any degree is okay, so my advice would be to go with something more marketable. However, you're still young, and haven't taken even a single college class yet - so just remember to keep your plans flexible.
London is not a country