Please tell me to what problems this can lead or how safety is compromised in a dual language ATC airspace. We already had this discussion on the hamster wheel-like " French ATC" thread. The safety argument of monoglots upon investigating had very little basis and it turned very ugly on both sides with the usual below the belt and nationalistic arguments.
I would say loss of situational awareness when some pilots understand the ATC and some don't.
Let's say a guy is reporting windshear in a different language and you are 3 NM beind him or maybe he's turning without ATC's approval and the ATC is trying to call him in french and you are not paying extra attention because you don't understand what is going on.
Or maybe you are on final at around 4 nm and the guy is authorizing another traffic to cross the active runway (happened to me in Spain) and i had to go around since at minimums i just saw the guy in the middle of the runway at the first intersection moving extra slowly.
The list is long and I think speaking the same language IS an increase in air safety.