Traditional Chinese Characters
Would like to see both Traditional and Simplified Characters used.
For now only Simplified are used. Due to the ongoing demand there's a chance Duolingo might expand the Chinese course to teach Traditional characters.
This is on Duo's "would be nice to have" list, but you can be sure it won't happen anytime soon.
But you can try, for example, ChineseSkill, HelloChinese, Decipher Chinese, or Du Chinese, which are all Chinese learning apps that you can set to display either simplified or traditional characters.
Decipher has a web version as well as a mobile app. I believe the other three only have mobile apps.