The question "When are you in Berlin?" sounds, somehow, strange to me and informal. Is it OK to ask this way in a business environment or is there a better way?
What do you mean to ask?
When are you in Berlin?
This carries a little of the connotation that the person being questioned either has regularly scheduled visits to Berlin (response: "Every April and May") or has a longer itinerary already planned out, in which Berlin is a stop. (response: "After Holland and Zurich, but before Warsaw and Nagurskoye.")
If you are asking someone who is going to Berlin as a one off event, and not in the context of a longer trip, either
When will you be in Berlin? or
When are you going to Berlin? are more common. (AmE, West Coast)