When Mon-El arrives on Earth, he’s in a Kryptonian pod. He then spends a while in a coma. When he wakes up, he apparently speaks English, though he has some trouble with certain phrases:
MON-EL: This is awe and some.
WINN: No…no, it’s just awesome. You gotta put those two words together, you’ll be golden.
Mon-El also implies that he learned English:
MON-EL: I’m sorry, English is my second language.
Kara and Kal had the benefit of an upbringing by human families, who (living in the United States), could well have taught them English, but Mon-El didn’t.
Did Mon-El already know English? Do most aliens? And if so, how? (Perhaps super-telepathy?) Or did he learn it quickly somehow?