Please can anyone tell me the langauages needed to build a desktop app
Languages like Java and Python have GUI libraries to build the desktop applications. Java has JavaFX and Swing. Python has PyQT, for example.
What are these desktop applications? Some examples in Windows are the clock, the browser, notepad, file browser, calculator, a programming editor / IDE, iTunes, etc. GIMP is an opensource desktop app written in C.
What language? You can learn any programming language (or languages) to develop these apps. You may as well research little more about the kind of desktop app you want to write and the features the app may have and look for a language that has the capabilities. Each programming language has its own advantages and capabilities.