As I understand it, when Alexa will have launched in the EU, it will be possible to change the language in the settings to German.
It should be possible to also use English without reversing the changes again, though.
Especially in Germany, it will take a while until there are a lot of German skills and the voice input/output will be equivalent to the English version. And I guess many heavy users speak English and want the English functionality, too.
Like Google Now, it should be possible to talk in English and German to the Echo (I guess Google Home will provide that functionality ;) ).