No clue what fitbit is up to.
Feel free to add your suggestions to the 'Feature Requests' Sub Forum, where users can vote, add comments,.. if there's a certain interest then there's a chance the developers pick it up and it'll be developed.
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Welcome to the Community @Yuehyao! As @SunsetRunner mentioned, currently I don't have any information as when this language is going to be supported. However, you may want to vote for this idea Traditional Chinese. Those ideas are viewed by our engineers and developers.
Thanks for voting. 
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