For creating an app how many and which languages are important to learn

Answer 1

It depends on a platform and type of application.

Answer 2

java is the best for creating apps :)

Answer 3

You only need to learn 1 language but you will need to learn the relevant API's for that language to do most things.
for android most people would say java, its probably the easiest for the job.
I think you need object-c for ios using xcode (idk much about it).
I dont know much about other languages but c++ can create IOS/Android/Windows phone apps but its complicated(and a lil faster).

edit:
Xcode according to Wikipedia:

"Xcode supports source code for the programming languages C, C++, Objective-C, Objective-C++, Java, AppleScript, Python, Ruby, ResEdit (Rez), and Swift, with a variety of programming models, including but not limited to Cocoa, Carbon, and Java. Third parties have added support for GNU Pascal,[5]Free Pascal,[6] Ada,[7] C#,[8] Perl,[9] and D."