I've worked in a multi-language business for many years where I did not speak all the languages, and there are two points I think you need to take to heart
1. No one can guess what you're feeling/thinking, you need to tell them
2. Don't take it personally if people are doing what comes naturally to them
Your comment on benefit of the office is understood, but misguided; your colleagues don't have an obligation to speak a specific language to you and make you feel better, that's not their job. I am not trying to be harsh, but you need to understand that your personal desire and comfort is not their responsibility. If they aren't being hostile to you or any other workplace violation, they're not doing anything wrong.
My guess is you feel left out because you _don't know_ whether you're left out or not. Even if they talk about your project in Mandarin, can you be 100% sure it's something relevant to you? Or even really about the project? Can you be certain that one of your colleagues wasn't just talking about how stressed they are about something and just wanted to vent? I'm not sure you can know this unless you ask them or learn the language, and in fact, both will help you feel a bit more included.
Overall, just talk with the colleagues in a private way and just mention your feelings and how you really want to join the conversations with them on the project but cannot if they converse in Mandarin; after that it's up to them.
For yourself, understand that you don't need to be part of every conversation and your colleagues don't have an obligation to you or the company to hold every conversation in English. The steps you can take are to just calmly tell that you want to join the conversations about the project (or others), and also learn some Mandarin. It's honestly not _that_ hard to pick up languages at least at a rudimentary level. Probably you will sound funny at first, but everyone does with a new language. Use it as an icebreaker to get in and just own it, and soon you will all find a middlepoinn that works for you.