I think it's less about how cabin crew address premium passengers, rather how they recognise premium passengers.
If LH English-speaking business class (sometimes first class) passengers are still being addressed in German at the end of an 11 hour flight, there's something wrong.
Not every time, mind you and on occasion, they're up there with Singapore Airlines in attentiveness and general clued-upness.
As in: Referring to the passenger list during the service, inquiring as to the correct pronunciation.
I'm not that worried about "Mr" this or ""Mr"" that - I just don't want to hear "Was möchten Sie trinken?"" after answering in English for the previous 10 hours.