To volunteer as a marshal for the DTM series some working knowledge of the German language may be required. However I have come across English speaking marshals that have worked DTM events with ease as the German marshals are particularly accommodating.
There are of course DTM events run outside Germany. Where for example to marshal at the British DTM event you ought to be a member of one of the clubs there. Contacting the circuit where the event takes place may do you little good as you must be an established marshal, so going the club route may be more appropriate.
I have very little information on marshaling DTM events as I have not done it myself, so any information anyone would like to share on this subject would be extremely welcome.