I don't think my folks, being exprienced travellers, will mind the plane change. They survive ok communicating in English but I kinda doubt they would pay that much attention to an announcement in the plane, and being honest, that they would be able to understand it either unless carefully concentrating (yeah, right, how skilled do you have to be to understand "Welcome to this NW flight to Los Angeles..."). However, most likely, a check-in clerk at HNL will explain them - nice and slow - how their flight will proceed and an extra stop will not eventually be a surprise. I'm pretty sure though that this "extra" manueuver will be a piece of news to them - dad didn't seem to have a clue about it when we went scrutinously through their travel plan. Neither did I realize it at that point - the HNL-DTW 11:40 flight time seemed oddly long, though. Had the a/c been a 747 instead of a DC-10 I may not have pondered this even this much. Sure, the ER model flies long legs but this just seemed not right, especially as NW has even more suitable long haul a/c in its fleet.
What amazes me is that the travel agency was not able to inform about this. They gave a print-out that nicely listed every leg of the journey (HEL-AMS-LAX, LAX-HNL, HNL-DTW-AMS-HEL) with ample information, and this very easily mislead one, me included, to think they'd fly from HNL to DTW nonstop, or as I later figured, perhaps with a brief stop for pax and fuel for example at SFO. An actual plane change NEVER occured to my mind.
I really didn't think about all this other than in trivial sense but true, this kinda of situation could potentially cause lotsa gray hair and grief to people with little travel experience and limited language skills. I personally see the ticket seller responsible for providing complete and accurate information regarding the flights, stops and plane changes. I wonder how much problems this kind of issue causes? Anyway, an excellent point has arisen from this discussion, thank you all for participating.