"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? "
And do you really believe that ATC using a dual langauge in these environments is safe.What happens when something goes wrong.Will he/she remember which langauge they are supposed to be using to the a/c with a problem or the two they are trying to give avoiding action to.Under an extreme pressure situation I very much doubt it.
Dual langauge may be "safe" in a very quiet ATC sector but in a busy sector in my humble opinion its just an accident waiting to happen.