""Language is no more than a tool... like any tool it has to be effective in its task and it has to be used correctly..."
That's the point..using correctly a language that is not yours is not easy...before removing all the other tools, be sure everybody will be able to use the remaining one!
It would be interesting to know if Germans ATC have comprehension problems with German pilots...do they use their mother tongue sometimes?? Do you know if German pilots speak english in the cockpit?