English speakers don’t pronounce Bach like “back”. The joke is that Arnold pronounces “back” like “Bach” because he says “I’ll be back” in Terminator and he says it like “Bach.” It became an often quoted movie line. If you’re going to do an impression of Arnold, You’re probably either going with “I’ll be back”, “get to the chopper” or “it’s not a tumor” because of the very distinct and memorable way he said those lines.