Leaving aside the regional dialect that might cause someone to say "on line" or "in line," isn't the line what establishes who the next customer is -- and so therefore redundant? Wouldn't a simple "May I help the next customer?" suffice?
I know line-forming is not something done in all cultures, but I'm fairly certain it is in most predominantly English-speaking ones. We form lines without being told or prompted. Even very small children can do this.