Technically, Saber shouldn't even know modern English.
Well they did say the Grail did teach them a couple of things about the modern world, such as languages, how to drive and how to eat with chopsticks, among other things.
That's why you won't see any Servants gets down on their knees and starts worshiping a TV as a "miracle box" or something.
But then again Kiritsugu grew up outside of Japan, lived most his life traveling the world and Kirei also spent a good deal of time overseas and also married a foreigner, so I doubt they'll actually have any issues with English.
azurelorochi said: But then again Kiritsugu grew up outside of Japan, lived most his life traveling the world and Kirei also spent a good deal of time overseas and also married a foreigner, so I doubt they'll actually have any issues with English.
In canon anyway. So honestly, when it comes to most characters in any anime that is supposed to speak perfect english, blame the voice actor with their japanese accent instead of story writer.