There is NO world wide warranty on iPhones (or iPads)
However Apple treats the EU as one country for warranty purposes so a German iPhone is covered in Belgium
There is only warrantee in the country that you bought it in. Also in European countries (because the provider systems are not "homogenised").
What is interesting though that the EU made it obligatory that you have TWO years of warrantee. Apple is slowly complying to that. In BE you probably have the two year warrantee. I can imagine that you want to buy in DE because the VAT is seriously lower there than in BE, but you will not have warrantee in BE then.