Response from a German on German language games
Being in Germany, we perceive the German versions to be something a bit more special. I personally assume, that less then half of all machines of any particular type featuring German localization were made were German - thus they should actually be relatively rare.
Over here in Germany, about half of what is seen in collections and on the market for my favorite Bally EMs from the late 60s and 70s, appears to be German language. Thus being "rare" seems to apply mainly ouside Germany.
Somehow, I compare this with some "Italian" games - which in contrast to the German ones often had a different title and in principle count as separate games. Here the appreciation seems to be a bit more obvious.
Paying the same price for a "rare" game looks like a gap in market pricing: typically people pay a pretty serious premium for rare games - where this seems to be an exception/opportunity?!
Axel