Where is Aa/Яя button?
I try Dolphin, Chrome and LG browsers on Mobile & Firefox, IE & Chrome on Laptop?
Oh my God! "After you begin the lesson" was the Key. Thank u WanderingMonk. In fact, you do not need to go to a desktop to change this preference. It is just the same if you go to "View Full Site" in the options and start a lesson. The button Aa/Яя appears (but if you use a smartphone like i do, you´ll probably switch back or restart the Duolingo web page for a more confortable view.