Hello,
Those languages are officially supported by Telegram and are maintained by a team of volunteers. Hence, they are appearing in the list of officially supported languages. As soon as Chinese Traditional becomes 100% translated and is of good quality, it might made it to the list.
https://www.transifex.com/telegram/public/
At the moment, I am not aware of any volunteers that are working on the translation. If the creator of @Tele_zh_TW is willing to help, (s)he can login to our Transifex page and improve the strings. 😁