Right you are! And in 'The Good The Bad The Ugly' actors Clint Eastwood, Eli Wallach and Lee Van Cleef were the only actors speaking English. Even the director Sergio Leone did not speak English.Another argument for shooting silent and ADR'ing all the dialog was that in Europe especially, films were routinely dubbed into a dozen languages for release in different regions. If 90% of the audience was going to see the movie with dubious lip sync and different actors doing the voices anyway, there wasn't a big extra incentive to record quality dialog on-set just for the minority who would view it in the original language. It sure saves time and money to not worry about vocal delivery, room acoustics, traffic noises and boom shadows in every setup.
And with mixed international casts and crews, there may not even be one definition of what any film's "original" language was anyway. It may be better to replace lines than try to dig meaning and emotional nuance out of the heavily accented speech of an actor struggling with a language that is not their own.
- Greg