Does hiding a card trigger Legion?

No, hiding a card does not trigger Legion.

According to the official rulings and the Comprehensive Rules:

• Hiding is not Playing: Rule 737.1.c.1 states that "Hide is not a subset of Play." Because Legion specifically requires you to have "played another Main Deck card" (Rule 738.1.c.1), the act of placing a card face-down does not satisfy the condition.
• No Chain: Rule 737.1.c.2 notes that "Hiding a card does not open a chain," further distinguishing it from the act of playing a card.
• Playing from Hidden: While the act of hiding the card doesn't trigger Legion, playing that card from its face-down state on a subsequent turn does count as playing a card and will trigger Legion effects for other cards played afterward that turn.

Summary:

• Hiding a card: Does not trigger Legion.
• Playing a card from Hidden: Does trigger Legion.