Are Japanese edition cards allowed in the main tournaments?
No, only English edition cards may be used in the main tournaments. No mixing of multiple languages in the deck is allowed too.
If I do not reside in the region(s) that the tournament is held for, may I still participate in them? i.e. I live in the AO region but would like to participate in the EU or NA/LA time zone tournaments.
Yes, you are allowed to participate in any of the tournaments. However, please note that the tournaments for each region will be happening at different time zones and the swiss rounds may be run concurrently. Please kindly refrain from pre-registering for all 3 different time zones if you are not able to be present for the check-in and tournaments.
Am I allowed to switch decks and/or change cards in the midst of a tournament?
Participants are NOT allowed to switch decks or change cards in the tournament, except for what has been stated in sideboard ruling for Future Card Buddyfight.
Should I sleeve the cards? May I use different sleeves for G units (Cardfight!! Vanguard) / Buddy Monster and/or Flag (Future Card Buddyfight)?
You may choose to sleeve your Imaginary Gift Markers. However, if you do, you MUST use different card sleeves from your main deck for your Imaginary Gift markers.
You may use different card sleeves from your main deck for your G units.
Future Card Buddyfight:
You should use the same card sleeves for the whole deck, including main and sideboard cards, as well as the Buddy Monster. However, you are allowed to use a different sleeve for your Flag card.
You MUST use different card sleeves for Lost World Deck in order to differentiate your Main Deck from your Lost World Deck.