When the app is first launched, you are given the option to download the data file for a language of your choice. Later, if you decide to check out other languages, you can manage the data files in the Settings screen.
In the letters tab of the app, you can choose the language you wish to learn from the " Language" drop down. The app will now display all the characters of the selected language. From the "Convert To" drop down, you can choose a language you know. Now, you can tap lightly on a character to see what it looks like in the second language you chose (the one you know). Tap longer (touch and hold) on a character to see more detailed information about that character.
Here, you can copy in text in one of the languages for which you have downloaded data files for and transliterate it into a different language of your choice. Note, the app tries to preserve and use the corresponding letter of the second language where possible (even if the a letter is pronounced different in that language). The idea here is to help the user recognise the letter in the source script.
To use the practice tab effectively, it is recommended to install a keyboard that supports at least two languages - one that you wish to learn and another that you know. I recommend the "Indic Keyboard" by "Indic Project" available in the Play Store and F-Droid. In the practice tab, choose a script you wish to learn and a script that you know. Next you can choose a suitable practice type and test your knowledge by entering characters in the script that you already know.