Hey I would like to use Duolingo in Mobile and connect it to a projector...
Android mobiles: via a "Google Chromecast".
Please, have a look at the different possibilities by searching in Google for:
phone connection to projector
Buy a Microsoft Display Adapter. We use that at my company conferences for projecting any phone or laptop to a projector or TV. As long as your projector has a USB and HDMI plugin it will work. It does not work with iPhones though. I use one at home as well with my TV so that I can cast anything from my phone or laptop. Transfers the sound as well. You can find refurbished ones for really cheap.