Yes, I thought it was likely the English mono mix was completed first. According to LDDb, the German THX edition is in Dolby Surround, which is potentially more useful than it being in mono. But even if you decoded it to 4 discrete tracks, and found that all the (German) dialogue was in the center channel, you'd have a hard time filling that dialogue back in, as the only available sources of the alternate (English) lines are in mono (i.e. mixed in with everything). Not to mention the fact that the dianoga cue and the Dave Schnuelle quote prove this mix is different from the English mono mix. You could make a mono version of just the non-dialogue parts, but even that would be different from the original mono mix, and the quality change when switching to dialogue would be very audible.