I guess that the data volume very much depends on your phones camera resolution. Thus, a single OCR request can easily add up to 1MB, and I would discourage use of OCR translation apps when on a volume data plan. However, there really is a huge number of mobile providers who offer prepaid data flatrates (usually up to several hundred MB a day) for as little as 2 EUR per day, or 30 EUR per month. Signing up is easy, just walk into a store (don't know whether there are mobile phone stores in DUS, I'm already airside as I'm writing this) and ask for a suitable plan. This should also work without knowing a single german word.