Aircraft registration was DM-XXX first and changed to DDR-XXX later (don´t know exactly when but must been in the 80´s). Finally registration was D-XXXX after 1990. . .Knowing from east german films and an east german television soap opera about the interflug (don´t remember the title but it was repeated in television two years ago and I found it very interessting) official aviation language was english.. .Some more details might be found in a book I read about two years ago taken from the library so I can´t look into it again to answer your questions ´Weg und Absturz der Interflug - Der Luftverkehr der DDR´, Brandenburgisches Verlagshaus, Berlin 1994 ISBN: 3-89488-071-6 DM 39.80 (unfortunately in german language) or in this
PDF-file where the old and the final registration is shown.. .Instrumentation of the Interflug planes was with metrical altimeters, so I assume flight levels were generally given in meters in all eastern block countries.
I´ll e-mail you more information if I find some.