Can I redeem a 50 EUR iTunes gift card purchased in Germany in any other country of the Euro zone?
No. You can only redeem a gift card purchased in Germany in Germany. See this Apple doc, the section called "GESCHENKGUTSCHEINE, ITUNES KARTEN, GUTHABENKONTEN UND CONTENT CODES".
Google translate does a very nice job (the doc is in German).
Anyway, here's the (translated) quote:
Gift certificates, iTunes Cards, Content Codes and credit accounts that were purchased in Germany can be redeemed through the stores only in Germany.
It appears that this is the case for most countries.