This may sound like a stupid question but I’m genuinely wondering so bear with me… I’m an Italian student and I’ve been contacted by a college to arrange an interview. The alumnus who contacted me is also Italian. Is the interview going to be in Italian or English? Should I greet him in Italian? I’m mean it seams strange to have an interview in Italy with an Italian person and speak in English but I’ve also read somewhere that interviews are supposed to test your English language skills, so what do you think?
I think you can follow his lead. More than likely you will exchange a few pleasantries in Italian and then he will say, “Let’s switch to English if that is OK with you.” Which, of course, it will be. 
Obviously if he greets you first in English, then respond in kind.
If the alumnus attended your future English speaking college, they should be able to speak English. So do what @skieurope says
Most likely English
English.
My daughter’s two interviews (in Turkey, with Turkish alumni of two US schools) went exactly as described by @skieurope - some small talk in Turkish, the bulk of the interview in English. She said both switched back to Turkish when it became apparent that her English is excellent…