Duolingo: Sprachkurse umsonst
The above is the headline of the duolingo app in the google play store. While it IS a correct translation into german ... I still wouldn't use it that way. Why?
"Meine Bildung war kostenlos, deine war umsonst" is a rather well-known wordplay in german, which is often used to point out the difference between "kostenlos" and "umsonst" to non-natives. Both can mean "for free", but more importantly "kostenlos" literally means "without costs" while "umsonst" also means "for nothing / naught / in vain"
So, if you heard " My education didn't cost anything, while yours was in vain" ... would you want to get an app advertising itself as:
Duolingo: language courses in vain?
So yeah, maybe change that headline? And maybe include above saying to the german courses to explain said difference?