Are you now allowed to use Chinese cards mixed with English in tourneys or events? Such as buying Chinese seals?

Yes, you are allowed to use official Riftbound cards of any language (including Chinese) in your decks during tournaments and events, provided they meet specific conditions.

According to Rule 420.1 and RiftJudge FAQ #8434, you may mix cards of different official languages within your deck, including your Main Deck and Rune Deck. However, you must adhere to the following requirements:

• Physical Indistinguishability: All cards in your deck must be in identical, opaque sleeves. They must not be physically distinguishable from one another (e.g., by card stock thickness, weight, or texture). If a judge determines that your mixed-language cards are physically distinguishable, they may rule the deck as having "marked cards" and require you to make changes.
• Rune Deck Consistency: While you can mix languages in your Main Deck, it is recommended at high-level competitive play to keep each shuffled zone (Main Deck, Rune Deck) in a single language to avoid potential marked card issues.
• Non-Shuffled Zones: Cards in non-shuffled zones (Legend, Battlefields) can be any language without concern, as they are never randomized.
• Translation: You may call a judge at any time to receive the official English card text if there are translation questions or disputes regarding a card's wording.

Regarding your specific question about Seals: Yes, you can use Chinese-language Seals in your deck alongside English cards, provided they are sleeved in a way that makes them indistinguishable from the rest of your deck.