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".
Anyway, here's the (translated) quote:
It appears that this is the case for most countries.