As easy as it is be cynical about this, I can guarantee they wrote at least one alien language for Destiny. I once had a conversation with Marty O'Donnell about how difficult it was to write a language for the Covenant in Halo Reach, and how he was saddened to hear the players didn't even notice or really care after all the work was put into doing so. You're free to ask him yourself about this. ODST and Reach were both used as test-beds for Destiny. If they wrote a language for Reach, then it follows that they wrote a language for Destiny. Not that much of a stretch.