Why does the main deck need to be uniform language throughout, but runes, battlefield, and legend don't have to match the main deck's language?

Ruling: The main deck must be uniform language because cards from different language printings are physically distinct due to manufacturing differences, which would count as marked cards. Runes, battlefield, and legend don't need to match the main deck's language because those cards are never shuffled together with the main deck.

Nuances:

• Runes must be uniform language within themselves (can't mix language runes)
• The language rule is fundamentally about preventing physically marked cards, not about language itself
• Legends can even be in unwieldy sleeves since they're not shuffled into the deck