WEBLUEIT: I think you summed it up nicely. Situational awareness is becoming more and more important both for controllers and pilots. Having accidents like the one in Paris and to some extent Linate because of use of 2 languages on the air is silly. It's one of the things that is cheapest and easiest to fix.
I have worked as a controller in Sweden, where the military and GA used to use Swedish on the air. The military started transition to english in 1999, and have completed it now I think (They still used swedish in basic training in 2002). The GA might take longer, but the percentage of swedish speakers on the air have decreased.
I found it a big pain in the ass to keep track of who spoke what, and offcourse had the problems as stated by others regarding SA.
Cheers