As far as I can see, Amazon MWS doesn't do currency conversion when it comes to product pricing - I couldn't find any reference to exchange rates in the docs. So you will have to set those prices yourself by explicitly setting a EUR price. That also gives you the opportunity to decide when to react to exchange rates and to pick a good price point. Today (Feb 21st 2013), 10 GBP = 11.55 EUR - it's totally up to you to go for 11 EUR, 12 EUR or may be 11.99 EUR. After all, you're targeting a different market by submitting your data Amazon.de - deciding upon price points is a very important step in doing so, apart from providing German descriptions, international shipping and support.
At the same time, I think that Amazon MWS should have probably rejected your price by giving an error or at least left the EUR price empty, but not silently "convert" it to EUR. I'd consider this to be a bug.
Edit: It seems there is only one way to find out which currency is expected by the MWS system, which is through the ListMarketplaceParticipations call. It will return a default country code, a default currency and a default language code along with the associated Amazon domain name.
For Amazon.com it should return US, USD, en_US and www.amazon.com
For Amazon.co.uk it should return GB, GBP, en_GB and www.amazon.co.uk
For Amazon.de it should return DE, EUR, de_DE and www.amazon.de
... etc ...
Unfortunately, you'll have to hard code the associated MWS data endpoints into your software:
CA - Canada : https://mws.amazonservices.ca
CN - China : https://mws.amazonservices.com.cn
DE - Germany : https://mws-eu.amazonservices.com
ES - Spain : https://mws-eu.amazonservices.com
FR - France : https://mws-eu.amazonservices.com
IN - India : https://mws.amazonservices.in
IT - Italy : https://mws-eu.amazonservices.com
JP - Japan : https://mws.amazonservices.jp
UK - United Kingdom : https://mws-eu.amazonservices.com
US - USA : https://mws.amazonservices.com
