Want to use Duolingo? (And maybe why you shouldn't)
So you want to use duolingo? Well sadly for our puposes we are out of luck with the mobile version. Duolingo only offers their course in simplified Chinese and lacks support for zhuyin. If you are a mobile user only, stop reading here - there isn't much you can do. However if you use the web version, you are in luck as we can use extensions to modify the content and make it work for us. You will find all the following extensions on the essentials page.
This is what it looks like with no modifications.
This is what it now looks like when we activate the NewTongWen Tang extension. This converts the simplified Chinese into Traditional Chinese.
Now when we activate the zhuyin extension, the zhuyin ruby characters get added next to the Traditional Chinese from NewTongWen Tang.
And finally this is it with Pinyin off. I think this is a functional method, though not super pretty due to the font used by Zhuyin (during my port I looked for a better font but couldn't get any sans ones working so for now this is the best I can do).
Now I wouldn't rely on this to work indefiniately, and I expect Duolingo will stop the use of extensions at some point - but for now it works if you are really set on using Duolingo for whatever reason. I personally would not start using it if you are not already deep into it as I feel that it really doesn't respect your time and is pretty inefficient and bloated. If it works for you however I won't judge! You do you :D