There is no actual Readlang app in the mobile app stores. But you can install it to your homescreen by following these instructions: Install Readlang on your iPhone, iPad or Android home screen
There’s also the web reader, but I warn you that this is quite fiddly to install and use on mobile at the moment: Install and use the Web Reader. One day I hope to improve the usability of the web reader and/or importer on mobile. For now, if it’s possible for you to import articles on desktop and then use your phone for reading and practicing flashcards, I would recommend that workflow since it works very well.