first of all thanks a lot for your translation. You are doing an awesome job! We have removed the support for gettext based translations because it was only a hack and did create some serious issues for us.
I have tried the languages.js modification and it does select zh_tw and display the Traditional Chinese string that I contributed before, the picture below:
I have also successfully yarn i18n-extract to update gsa-zh_tw.json to the latest language file to be translated, and I will start to work on the translation, thank you very much for your help!