On Github, some projects are tagged as being of one language or another. How do I add one of these tags. (Objective C, PHP etc.)
As far as I know, they aren't really tagged - the languages attached to them are the languages which you use in your project. Github looks through the source and figures out to the best of its ability what languages are in use.
If you go to your respository, then to the Graphs tab, you can see the language breakdown.