Unfortunately Google doesn't offer an automatic input editor for traditional Chinese like they do with simplified Chinese. Because of that, our default auto-editor is in simplified regardless of your account settings.
However, have no fear! FluentU's review system is fully-equipped to handle traditional Chinese. All you have to do is download, install, and use your own traditional Chinese input method editor when using FluentU and you're good to go.
Both Windows and Mac have their own traditional Chinese keyboard inputs available for use, and there are quite a few others available for download online.