Thank you for your quick reply, but that does not solve my problem.
The "Weiter (next)"-Button appears throughout the whole checkout process. When I change the translation file, all buttons will change but I need only the last button to be changend to "Kaufen (Buy now)". It makes no sense to have the other buttons changed this way.
This is a serious problem for all German users. German law forces us in internet business to make it exactly this way. There are lawyers who search the net for these things and if it is found you have to pay up to 1500,- €.