The German Federal Centre for Health Education offers donor cards in many languages specifically for this purpose. They do warn that the local rules apply by default (Grundsätzlich gilt immer die Regelung des jeweiligen Landes) but they claim the card would generally be honored.
Beyond the implementation details (opt-in, opt-out, central register or not, etc.) the rules generally give a strong weight to the wishes of the deceased. It does therefore make sense that carrying a document expressing those wishes unambiguously in the local language would make it more likely that your organs would be used following your instructions.