When I was a foolish High School student, and when the occassion called for a trip to a local restaurant to act foolish, our favorite thing to do was to take turns acting like an exchange student from some far off land, (where gibberish was the official language) and another as interpreter for things such as:
"Ishtildiga mayvenusti pothwalinte teshperinas"
"My exotic friend here will have the cheese fries, please. Also, it's his birthday, do you have any free specials?"
Oh how we amused ourselves. If only we were less foolish, and more evil, we could have started a religion based on pretending foreign exchange students had birthdays at local corporate restaurants. We would be awash in tithings of hot fudge brownie sundaes.
ETA: anecdotal, but I can relate to arthwollipot's observation that groupthink takes over the excercise. Our group, although speaking gibberish, seemed to speak with lots of 'hard g's', 'ish's', and ended lots of words in vowels. (The above example was a pretty accurate indicator of the style).
So, in a way, I get it. We all made up the same sort of BS, and we always knew what it meant: Cheese fries, birthday. (Amen, hallelujah).