The 'Buy Now' text uses this language constant
AMAZONWS_BUYNOW="Buy Now"
You just need to set up another language override for this.
I am puzzled by what you mean:
But i also need to have a german Price in EUR
If you are using Amazon.de then you should be getting the prices returned in Euros. If you are not getting this, what are you getting? Possibly you are making a mistake in specifying the country in the options, if the plugin does not recognise the country option then it will use the amazon.com website for security reasons. For this reason, to avoid mistakes, it is best to set this option in the plugin manager options rather than the embedding code.