> The fact that Reach was after 10 or more years of battling the Covenant (can’t remeber the exact date of 1st contact) and the events of Halo CE take place only a few short days after Reach then I would say that blows your theory.
First contact with the Covenant occured in 2524, and the the war broke out the next year. Thus the events of Halo: Reach take place in the 31st year of the Human-Covenant war. Just to clear that up.
Anyway, it doesn’t really matter what the in-game explantion is for humans and Covenant being able to understand each other (see note below). The point is that the Covies provided quite a lot of entertainment with their funny one-liners and dialogs since H:CE, especially with the IWHBYD skull enabled in H3 and ODST. And that part was unfortunately completely missing in Reach, sadly taking away at least some of the replay value the campaigns had provided.
Note: The Covenant were smart enough to realize that understanding humans was important, and so they trained countless Unggoy in various human languages. It’s actually uncommon for an Unggoy not to be fluent in at least a couple of human languages.