Jerricho,
good thought, but I disagree on the "percentage increase in air traffic movements in busy Terminal Environments".
I have flown into a few very busy airports in the US (ORD, ATL, JFK, EWR, BOS). When the ATC controller is so busy that his communication sounds like a non-stop AK-47, can you really picture all of his transmissions? I agree that it sounds strange to a person the first time he's exposed to dual-language ATC and that he feels unsafe. If you approach a very busy airport, bad weather, approach procedures etc. do you have enough time to catch all other communications? I think not. Will dual-language ATC make it more confusing? I think not.