Can you clarify your question or be more specific? Xcode is just an Integrated Development Environment, so nothing runs on it per se. Apple's languages of choice are Objective-C and Swift, although the compiler and various developer tools can be used with many different languages.
Out of the box, C, C++, Obj-C, Obj-C++, and Swift. As mentioned above though, it's just an IDE. You could write plugins to make it handle any language you want, although the plugin system isn't documented by Apple at all and there's only bits and pieces that people have reverse engineered, so I'm a bit doubtful you'll actually find any plugins to make it handle other languages.
There are also a bunch of programming languages (Scheme, Lisp, Basic, Lua, Squeak, etc.) with open source implementations in portable C or C++ code that can be compiled by Xcode. Then run them separately on your Mac, most from the OS X Terminal command-line, but a few are stand-alone Mac apps. Or you could use Xcode's built in debugger terminal to run them completely within an Xcode window.