Thanks for all the help. I also got a reply from the embassy in Berlin saying that a 'Unterschriftsbeglaubigung' (either from a Notary or a 'Amt') is enough. But they also said that the statutory declarations are not that important and documents that prove, that we live together (tenancy agreements, utility bills to the same address, ...) are far more important. We don't have that, because we have been more or less travelling together for the last 1 1/2 years and have always stayed with other people, family and friends), so those letters would be the main proof, besides joint bank accounts. Because we are not sure that that is what is considered living together in a marriage-like relationship, we decided to get married before we send the application. We wanted to get married anyway within the next year or so, so we might as well do it earlier and apply afterwards. Now we are working on what we need to get married in Germany.