Symphony D.
Deity
They spent most of the time fighting each other, not talking to people. It can be assumed they learned English along the way, and it being fiction and all, that the entire thing is presented in English so you can actually understand it, like pretty much all Sci-Fi that comes in English. You may as well ask "Why do the actors in dubbed Godzilla movies speak English?"But didn't they know English like as soon as they came to Earth? And why would giant robots watch TV?

And seriously. Junk Planet. What else would you do if you were a giant robot on a planet made of rust?