What are the programming languages that I need to know in order to develop an interactive mobile application like sololearn?

Answer 1

Java, XML, SQL + Web Languages

Answer 2

You can pretty much do it in any language but you going to suffer from pain with some of them since some people didnt create/release android development kits for certain languages. (Or i didnt search hard enough)

So simpler ones:
Java (and kotlin) since you have android studio
C#/F#/C++ as you have Xamarin (in visual studios)
And some other languages that already have a android development kit released.

You need XML to design your U.I, but thats prob the easiest to learn.

Answer 3

To add to prev answers. You can make such app even with C++ using Qt framework with a great UI markup language - QML. Very similar to JavaScript an IMO more sane then css plus html or xml.

Answer 4

Each language has a particular framework that supports that. Search for each.