Not using a publicly appointed and sworn-in interpreter may prove to be advantageous seemingly because the legal (and financial) consequences of an improperly translated and thus misunderstood contract can ultimately cost you dearly. A foreign client’s claim to have a good command of German language should also be approached with caution because notarial recordings require not only good general language skills, but also a profound knowledge of legal terminology (in two languages, at the same time).
One recent example from my professional career: a customer intended to sign a marriage contract with his future wife and beforehand discussed thoroughly all provisions with her. At an “unofficial” appointment to clarify all possible legal issues (which fortunately took place before the actual notarisation of the deed), it became apparent that both parties could not agree on several regulations, despite the fact that they claimed to have had cleared up all disputed issues days before. Obviously, in the case of this Polish-German couple there were misunderstandings which arose from one partner's inadequate German skills and legal knowledge. After the complete draft of the marriage contract was translated into Polish and explained in detail during the appointment, the parties came to the conclusion that they wanted a different arrangement. If a qualified interpreter had not been present, then a potential divorce could not possibly have been avoided.