Looks like you’re already been a part of this project, since the 8th of January, this year. Have you tried logging into your account? Do you have any issues with that?
As far as I can see your current role in the NVDA project is translator so you can add translations or upvote existing ones.
I also noticed that Simplified Chinese is translated to 100% in the mentioned project, so maybe this is what confusing you. Even though it is completely translated you will can add your translations.