Is it necessary to combine the coding languages like C++ & Java to create good looks an application?

Answer 1

Sometimes, yes. It solely depends on what you are making most of the time. Python and C++ is a good mix.

Answer 2

If you mean "a good looks application", it is usually a problem, because coding GUI = graphic(window) interface with user belongs to the extended and deeper knowledges of programmic languages and libraries. Yeah, it is sometime a way to mix these libraries between languages, but sometime not. I am affraid, mix Java with anything else could be problem