Are there any HD Bitcoin mobile wallets with the mnemonic seed in traditional Chinese? If not, what would be the easiest way to modify an open source iOS/Android wallet (eg copay) to do this?
Here is the bip39 for traditional Chinese word list.

https://github.com/bitcoin/bips/blob/master/bip-0039/chinese_traditional.txt

Also, there are some useful pages that explain how to implement Mnemonic code for generating deterministic keys as following,

https://github.com/bitcoinjs/bip39

https://iancoleman.io/bip39/#chinese_traditional