Interestingly the “Close the Blast Doors” phrase was there from the beginning in the German Dub. At least my 1990 VHS TV Rip contains it (“Schließt die Panzertüren!”).
Correct me if I’m wrong, but weren’t the foreign languages dubs based on the mono track? If so, then that’d make sense.
Yes, it’s not easy to retrace why this specific phrase was included in the German dub, but on the other hand was completely absent from the english audio track. But it’s not the only audio anomaly in the German dub. As Adywan pointed out in this thread
http://originaltrilogy.com/topic/The-missing-Dianoga-music/id/52246
a specific music cue was included in the German dub, but once again was absent from the english audio track in the first place. So, yeah that is the question: which mono track was used by the german dubbing studio to create the audio track?