Is learning& practicing from this app is enough for having a good command on language ?

Answer 1

No. Need to know more commands from others​ program code or from separate sites. But this application is best to start with.

Answer 2

No, but it will help you to start, but you will have to go further either on other more advanced website or by yourself (trying to do stuff you actually can't for example).

Answer 3

SoloLearn + w3schools + codecademy

Answer 4

No you will need to access other resources. This app is a launchpad designed to help you get a handle on the basic syntax required. It is by no means exhaustive and there are many concepts it does not mention. .