Does the deck language requirement apply to runes and battlefields, or only to the main deck?

Ruling: There is no explicit "same language" rule. The actual rule is that cards in a deck cannot be told apart without looking at them (e.g., by thickness or feel). This applies to cards within the same deck zone.

Nuances:

• Chinese and English cards have differences in thickness and feel, so mixing them in the same deck zone would violate the rule
• It's unclear whether runes need to match the main deck language, but as long as runes are all the same language as each other (and thus indistinguishable from one another), they would likely be acceptable even if different from the main deck language
• No official ruling exists yet on cross-zone language mixing