Studying a new language, especially as challenging as Standard Chinese or Cantonese, is the most rewarding and mind expanding out of those three options. It helps with your native communication, memory, imagination, makes you read valuable books you were interested in reading anyway. Once you know two languages, your available perception of the world grows exponentially, it's something priceless to be able to think in two different ways so naturally.
Piano gives you greater dexterity, memory, ability to express emotions, kinesthetic and musical intelligence and possibly enables you to learn music theory and some composition down the line.
Reading depends a lot on what you choose, you may sink in fantasy or sci-fi novels and not really get that much out of it other than verbosity, entertainment and imagination.
Try the language, becoming bilingual is a great boon to understanding the world. Ultimately, why not all of those? It's not like you have to restrain yourself with your hobby specifics and all of these activities can benefit you greatly in very different areas.
You may also think of synergies, in everything you do you might seek to connect various areas of expertise. Maybe when you're practicing piano, reading music related literature would give you inspiration and sensitivity. Studying languages, you may also draw from wonderful foreign literature, or indulge your musical interests in foreign modes of composition or traditions. All those three interests can be connected.
Piano is the most resource restrictive out of the three, since you need to invest in the instrument and carry it around everywhere to continue with your practice. The remaining two aren't as equipment exclusive and don't force you into a dead-end financially and don't limit your mobility either. By dead-end I mean that once you commit it may be difficult to diversify your interests since whatever else catches your fancy could potentially make you regret your initial investment as a result.
