How to do Romanian from English lessons on the Android app
Until said course is released for the app, this is a workaround I've used with great success. However, it's only for strengthening skills and NOT for doing new lessons. (You'll still need your browser for that.)
This involves switching between your browser (mobile or desktop) and the app, and being VERY fast-fingered.
1) Open the Romanian from English course on your browser.
2) Open the Duolingo app on your mobile device. You must be able to tap the barbell (strengthening) icon the instant--and I mean the millisecond--it becomes available. It will not matter which course is already on the app, as long as you can tap the icon before you get the "The language pair you are currently learning..." screen comes up.
3) If you did it right, a Romanian lesson will load--with full functionality!--like with any course that's already on the app.
4) Once your lesson is complete, be ready to tap that barbell icon again once you hit CONTINUE. Again, if you did it right, you should be able to do another lesson.
5) Once that lesson is done, repeat the above step. But now you have to be so fast that you have to tap the barbell icon with no delay after you hit CONTINUE. (It helps to have your finger hovering just above where the icon will appear.)
6) And now you should be able to do a third lesson. But no matter how quick your reflexes are, you will be taken right to the "The language pair you are currently learning..." screen after the third lesson. You will be give a list of other courses to choose from, so pick a different one. You do not have to go through a lesson, just have it set on a different course.
7) Exit the app and repeat from Step 1. You might find yourself being able to do only one lesson to start, but you should be able to get to three in a row as you get quicker with your timing.
Once you have the tree completed, you'll only need the browser for setting the course.
I've tried this on iOS, but I can't cheat the app on there like I can on Android. Perhaps someone more familiar with iOS can do it.
I've also tried this method with the Greek from English course, but without success. It seems to work for just the Romanian course.
Hope this helps!