You can always select your preferred language when you first login to http://www.mms.t-mobile.de to view your MMS online. A welcoming screen showed:
You have received an SMS notifying your that a T-Mobile MMS message has arrived?
You can retrieve your T-Mobile MMS here.
To do so, please enter your telephone number and the password we sent you by SMS.
Then confirm with “Log in”.
Enter your telephone number (e.g. 016012345678 or +4916012345678) and enter your password (e.g. A1B2D3C4) here.
Quoted from Wikipedia:
T-Mobile is a mobile (cell phone) network operator headquartered in Bonn, Germany. It is a part of Deutsche Telekom and belongs to the FreeMove alliance.
T-Mobile is a group of mobile phone companies (all owned by Deutsche Telekom) that operate GSM and UMTS networks in Europe and the United States. The “T” stands for “Telekom”. Most subsidiaries (companies) of Deutsche Telekom have names beginning with “T-” like T-Home and T-Online. T-Mobile also owns parts of mobile operators in Eastern Europe. Globally, T-Mobile has 101 million subscribers, making it the world’s sixth largest mobile phone service provider by subscribers and the third largest multinational after the United Kingdom’s Vodafone and Spain’s Telefonica.