You can create a bundle name using any language you desire and you can even code in a foreign language if you like. In Mac OSX just make sure you add an input source supporting your language. (System Preferences > Keyboard > Input Sources) While in this area check that Show Input menu in menu bar is enabled. You can configure Shortcuts > Input Sources to leverage shortcuts if you need to move between one or more languages.
I don't believe that the default development language cannot be changed. For example if-then statements and all other language constructs cannot be changed. You can use the input source defined above to write your comments and variables in Greek (or any other language) if you choose.
I think that I would leave the English and Greek localizations since the English one will always be the system default (for new Xcode builds) and may lead to unpredictable results as Xcode continues to be ungraded.