how do you keep up grades when studying abroad when you don't speak the language
I hope you are doing well. I do really understand where you coming from. I think it is really stressful to do well and received good grades while the local language is you second language. Personally I had same problem during my college time, I had same difficulties and I think that's the main reason I focus more on subject like Math, Physics, Chemistry, Programming, and IT.
Try to get along with the locals and keep up confidence level, because I think that is foundation of keeping up with good grades.
Keep up and hope you navigate to resolutions.
Great to hear that you're going to venture out to continue your studies! How exciting! As far as you question is concerned, find out what language the course will be taught in. If it is in Japanese, what level will it be in? I studied abroad in Austria (where they speak German) and the teachers understood that my class, all from the US, was not fluent. The lessons were slow and the teachers gladly repeated themselves if we asked. Are you going to be the only American in your classes?
In the end, you are going to want to work hard to learn as much of the language as you can before you go. This way, you can interact with locals and being bilingual is a great skill to have on a resume. Duolingo is a good place to start, if you haven't already. It's free and will give you the basics of the language. https://www.duolingo.com/
Watch your favorite shows and movies in Japanese. See if you can take a professional course or get a tutor for a reasonable rate.
When you get to Japan, see if you can meet people who want to learn English. Create a "language trade-off" where you take turns speaking in one language, then switching to the other.
The more you learn in the language, the easier your transition will be in your new temporary home.
Hope that helps!