What language is used to design an app

Answer 1

you can use JavaScript to design an app too :D

Answer 2

Depends on what you want to make your App.

On Android it is advised to do Java, but you can also use C#
On Windows you can use Java, C#, C++, Python and the list goes on.

Answer 3

if you want design: visual paradigm is a useful tool for Object Oriented model. but if you want develop I recommend to you learn Java. after all is the most used language in the hole world