I got a reply!

Almost immediately. Very surprising.

マランダー　リーさん
連絡ありがとうございます！
オーケストラの参加もちろんＯｋです。
歓迎します！
さっそく明日、練習があります。
ａｍ10:00から
場所は、龍郷町りゅうゆう館です。
また今度、英語の話せる人からメールを送ってもらいます。
奄美オーケストラ

No translation accompanied the reply, but here's what I could piece together using what little I know of Japanese, with the help of Google Translate. (It's probably inaccurate.)

"Maranda Li-san, thank you for contacting us. Yes, it's ok for you to join the orchestra. Welcome! There is rehearsal right away tomorrow starting at 10 am at Ryuuyuu Hall in the town of Tatsugo. Soon we'll have someone who speaks English send another message. - Amami Orchestra"

In the next few exchanges (which remained in Japanese), they offered to let me borrow a beginner's cello until I could visit the string shop on the mainland to rent my own instrument.

I also asked about their upcoming program, which features movements 3 and 4 of Beethoven's ninth symphony (!!) as well as traditional island songs, anime theme, and other fun things. They rehearse Sunday afternoons, from 1:30 to 4:30 pm, and are based in the main city on the northern half of the island, 26 miles from Setouchi. Still, thanks to winding island roads and the slow pace of island life, it takes 50 minutes by car or about an hour by bus to reach it.

This will be a great opportunity to get to know locals while doing engaging in a shared activity; after all, music is the universal language. Grassroots internationalization at its finest.