So, if I want to become a French translator in the future, what subjects should I study in grade 9 and afterwards?

This depends on what your high school offers and what kind of translation you'd like to go into. Any language learning courses (ex. French, Spanish, German) are a huge help, and many universities require a university English Literature credit. Aim for other courses that help you understand culture, motivation, and social habits such as psychology, sociology, anthropology, philosophy, world religions, and global business. Your high school should have a counselor you can go in to and talk to; they'll have a better idea with what courses your school provides and what will be best for you!