Bumblebee demonstrates a capacity to use radio signals in order to (re-learn) English when his vocal unit is damaged by Decepticon fire and subsequently replaced. Note that this would also strongly suggest that he's not speaking English, but rather his vocal unit is translating spoken Cybertronian into English.
Charlie and Memo stared at the radio, then looked at each other. Charlie returned her gaze to the radio and said, “Whoa, wait. Is that why you’re always messing with the dial, Bee? Are you teaching yourself to talk?”
Bumblebee readjusted the dial to play the lyrics from a song by the Four Seasons. “Walk like a man, talk like a man.”
“No freaking way,” Memo said.
“Bee, you’re a genius,” Charlie said. “An actual genius!”
Bumblebee: The Junior Novel
Given that someone traveling at greater than the speed of light would have access to 50+ years of radio traffic, it's certainly possible that he taught his voice unit English in much the same way when he first arrived.