English usually, if people want my software in another language they can fork and translate it since it's usually open source.

 

C or C++. C for simple stuff, C++ if you plan to have an elaborate UI or multiple screens or threads for your app.

 

C or C++ should be prefered over every interpreted language (python and javascript) because the interpreter could contain bugs and limitations that are not in libnx and libnx already has enough problems.