The first time Project64 is started, it asks you to choose a language. The choices available depend on the contents of the \Lang folder. Project64 v1.5 shipped with a reasonable but of cause far from complete set of translations included (thanks to everyone who made them!). If the language you want is not in the list, seek an updated/alternative file or make/edit one yourself!

To change languages at a later date, use the Language menu under the File menu in Project64. Simply click an entry in the list and you should see the GUI text update immediately.