Never ending debate.
As some posters have already mentioned, and Vilters apparently fails to understand, is that pilots talking to ATC in their native language will have a lower probability of misunderstanding - this increases flight safety.
On the other hand, and obviously, those who do not understand the local language will lose situational awareness - this decreases flight safety.
So where does the balance between this increase and decrease in flight safety end up? Positive or negative? No one can pretend to know for sure, as it depends on the situations.
In my opinion, in controlled airspace the use of local language is beneficial (unless the local English proficiency level is excellent) while in uncontrolled airspace, local pilots should make an effort to switch to English whenever a pilot gets on the frequency in English. At least for those in his vicinity.